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THE  SOIL  SOLUTION 


Published  by 

The   Chemical   Publishing   Co. 

Eastern,  Penna. 
Publishers  of  Scientific  Books 

Engineering  Chemistry  Portland  Cement 

Agricultural  Chemistry  Qualitative  Analysis 

Household  Chemistry  Chemists'  Pocket  Manual 

Metallurgy,  Etc. 


The  Soil  Solution 


The  Nutrient  Medium  for  Plant  Growth 


By 


FRANK  K.  CAMERON 

In  Charge,  Physical  and  Chemical  Investigations,  Bureau  of  Soils, 
U.  S.  Department  of  Agriculture 


EASTON,  PA.: 

THE  CHEMICAL  PUBLISHING  CO. 

1920 


LONDON,  ENGLAND: 

WILLIAMS  &  NORGATE 

14  HENRIETTA  STREET,  COVENT  GARDEN,  W.  C. 


^  7  8  rs  <1 

tl  s  O  *J  3 


COPYRIGHT,  1911,  BY  EDWARD  HART 
COPYRIGHT,  1920,  BY  EDWARD  HART 


Preface. 

It  has  long  been  the  custom  to  regard  soil  chemistry  from  one 
of  two  diametrically  opposed  points  of  view.  Either,  it  has  been 
considered  extremely  simple,  or  complex  and  hopelessly  difficult. 
In  either  case  the  impression  has  generally  prevailed  that 
practical  work  in  soil  chemistry  consists  in  treating  the  soil 
with  some  solvent  or  other  and  analyzing  the  resulting  solution 
for  "available"  plant  food  elements;  in  other  words,  that  the 
chemist's  role  in  soil  studies  is  merely  that  of  an  analyst. 

Soil  chemistry  is  complex,  but  not  by  any  means  hopelessly  so. 
Unfortunately,  the  complexity  of  most  of  the  problems  presented 
has  deterred  the  student  of  pure  chemistry  from  attacking  them, 
and  because  they  do  not  offer  any  material  pecuniary  rewards, 
they  have  not  appealed  strongly  to  the  investigator  in  applied 
chemistry.  Investigations  in  soil  chemistry,  for  their  own  sake, 
or  for  the  sole  purpose  of  increasing  the  sum  total  of  human 
knowledge  concerning  the  phenomena  taking  place  in  the  soil, 
have  been  comparatively  rare.  The  subject  has  generally  been 
regarded  from  the  analytical  point  of  view  and  as  incidental  to 
agronomic  studies. 

One  purpose  of  this  little  book  is  to  show  the  investigator  in 
chemistry  who  is  not  limited  by  the  condition  that  his  work  must 
bring  some  personal  financial  return,  that  the  soil  and  its  prob- 
lems offer  a  field  for  his  efforts  quite  worthy  of  ranking  along- 
side the  most  interesting  branches  of  pure  chemistry,  as  well  as 
being  of  the  very  highest  importance  to  the  development  of  the 
welfare  of  the  human  race.  Another  purpose  is  to  point  out 
the  line  of  attack  upon  the  problems  of  soil  chemistry  which  at 
this  time  offers  the  largest  opportunity  for  results.  In  how  far 
the  details  of  the  story  in  the  following  pages  are  correct,  time 
with  its  further  investigations  will  tell.  In  a  sense,  the  correct- 
ness of  the  details  is  of  secondary  importance.  It  is  of  the  first 
importance,  however,  that  there  should  be  a  general  recognition 
that  soil  phenomena  are  essentially  dynamic  in  character,  and 
that  the  investigation  of  the  properties  of  the  soil  solution  and 
its  relation  to  crop  production  is  a  procedure  certain  to  yield 
results  of  positive  value. 


iv  PREFACE 

Again,  it  is  a  purpose  of  this  book  to  make  available  for 
students  of  agriculture,  a  systematic  outline  of  the  work  so  far 
accomplished  in  this  particular  field.  It  is  to  the  students  of 
to-day  from  whom  are  to  come  the  investigations  of  the  near 
future  that  the  book  is  particularly  addressed.  Some  of  the 
details  presented  in  the  following  pages  are  matters  on  which 
opposed  opinions  are  now  held  strongly  by  different  authorities, 
and  to  the  unbiased  minds  of  the  coming  investigators  must  be 
left  the  decision  as  to  how  closely  the  truth  has  been  approxi- 
mated in  what  is  written  to-day.  The  field  of  effort  covered 
by  this  book  is  one  in  which  there  is  an  increasing  activity,  and 
new  facts  and  deductions  will  inevitably  bring  modifications  to 
present  opinions.  To  encourage  this  further  acquisition  of 
knowledge  is  the  main  purpose  of  the  book. 

The  material  brought  together  in  this  book  has  been  presented 
to  the  faculties  and  students  of  several  of  our  Agricultural 
Colleges,  in  the  form  of  a  short  course  of  lectures.  In  large 
part,  moreover,  it  has  been  published  in  Volume  XIV  of  the 
Journal  of  Physical  Chemistry.  To  make  it  accessible  to  and 
more  easily  read  by  one  familiar  with  the  progress  of  technical 
soil  investigations,  it  has  been  recast  in  its  present  form. 

It  has  been  assumed  that  the  reader  will  have  a  fair  working 
knowledge  of  the  concepts  of  modern  chemistry.  Nevertheless, 
an  effort  has  been  made  to  avoid  technical  terms  so  far  as  this 
can  be  done  without  undue  sacrifice  of  lucidity  of  expression. 
Free  references  have  been  made  to  the  bulletins  of  the  Bureau 
of  Soils,  U.  S.  Department  of  Argiculture,  because  they  are 
generally  accessible  to  the  American  student,  and  because  in 
them  will  be  found  detailed  discussions  and  bibliographical 
material  pertinent  to  the  subjects  outlined  here.  To  his  coworkers, 
the  author  is  indebted  for  many  criticisms  and  suggestions; 
and  more  especially  in  the  making  of  the  book  is  he  indebted  to 
Mr.  S.  C.  Stuntz. 

Washington,  D.  C. 
1911. 


Table  of  Contents. 


PAGE 
Preface    iii 

I.  The  Soil   ..! I 

II.  Soil  Management  or  Control 4 

III.  Soil  Analysis  and  the  Historical  Methods  of  Soil  Investigation  8 

IV.  The  Plant-Food  Theory  of  Fertilizers 16 

V.  The  Dynamic  Nature  of  Soil  Phenomena 18 

VI.  The  Film  Water   24 

VII.  The  Mineral  Constituents  of  the  Soil  Solution 31 

VIII.  Absorption  by  Soils   59 

IX.  The  Relation  of  Plant  Growth  to  Concentration 70 

X.  The  Balance  Between  Supply  and  Removal  of  Mineral  Plant 

Nutrients    75 

XI.  The  Organic  Constituents  of  the  Soil  Solution. 79 

XII.  Fertilizers    105 

XIII.  Alkali    no 

Index   127 


AN  INTRODUCTION  TO  THE  STUDY  OF 
THE  SOIL  SOLUTION. 


Chapter  I. 


THE  SOIL. 

The  soil,  or  that  part  of  the  land  surface  of  the  earth 
adapted  to  the  growth  and  support  of  crops,  is  a  heterogeneous 
mixture  composed  of  solids,  gases  and  a  liquid,  and  containing 
living  organisms.  There  are  present :  mineral  debris  from  rock 
degradation  and  decomposition;  organic  matter  from  the  degra- 
dation and  decomposition  of  former  plant  and  animal  tissues; 
the  soil  atmosphere,  always  richer  in  carbon  dioxide  and  water 
vapor  and  possibly  other  gases  than  the  atmosphere  above  the 
soil;  living  organisms,  such  as  various  kinds  of  bacteria  and 
fungi,  with  the  products  of  their  activities,  notably  the  "nitrogen 
carriers"  and  the  enzymes ;  and  finally  the  soil  moisture,  a  solu- 
tion of  products  yielded  by  the  above  components  and  in  equili- 
brium or  approaching  equilibrium  with  the  solids  and  gases  with 
which  it  is  in  contact. 

In  its  relation  to  crop  plants,1  that  part  of  the  soil  of  imme- 
diate importance  is  the  soil  moisture.  From  this  solution  the 
plants,  through  their  roots,  draw  all  the  material  involved  in 
their  growth,  except  the  carbon  dioxide  absorbed  through  their 
leaves.  The  soil  solution  is  the  natural  nutrient  medium  from 
which  the  plants  absorb  the  mineral  constituents  which  have 
been  shown  to  be  absolutely  essential  to  their  continued  exist- 
ence and  development.  And  from  this  solution  plants  some- 
times absorb  dissolved  organic  substances,  but  such  absorptions 
are  probably  adventitious  and  incidental  to  the  growth  of  the 
t  plant  in  a  particular  environment.  While  it  appears  certain  that 

1  By  crop  plants  are  meant  the  ordinary  green  plants  employed  in  agri- 
culture. As  is  well  known,  the  fungi  as  well  as  certain  parasitic  and 
saprophytic  non-green  seed  plants  obtain  their  nutriment  in  a  very  differ- 
ent way  from  ordinary  green  crop  plants. 


2  THE  SOIL  SOLUTION 

no  organic  substance  in  the  nutrient  medium  is  necessary  to  the 
maintenance  of  plant  growth,  nevertheless  organic  substances 
are  probably  always  present  under  natural  conditions.  They 
may  or  may  not  be  absorbed  by  the  plant  and  may  affect  it 
beneficially  or  otherwise. 

The  study  of  the  soil  solution  is  of  the  first  importance  in  the 
investigation  of  the  relation  of  the  soil  to  plant  growth,  and  in 
the  following  pages  there  is  given  an  outline  of  our  present 
knowledge  of  the  chemical  principles  involved,  with  such  dis- 
cussion of  the  physical  and  biological  factors  as  is  essential  to 
an  orderly  presentation  of  the  subject. 

To  understand  clearly  the  relations  of  the  soil  solution  to  the 
soil  as  a  whole  and  to  the  plant  which  it  nourishes,  it  is  desir- 
able to  consider  some  attributes  of  soils  in  general.  Every  soil, 
no  matter  of  what  type  it  may  be,  is  a  complex  system.  In  it 
various  processes  are  continually  in  operation,  excepting  possibly 
in  the  extreme  case  when  it  remains  frozen  for  a  time  at  some 
definite  temperature.  The  resultant  or  summation  of  these 
processes,  whether  expressed  in  plant  production  or  otherwise, 
will  vary  from  time  to  time,  both  quantitatively  and  in  direction ; 
for  instance,  as  to  the  amount  and  kinds  of  plant  growth  it 
produces.  That  is  to  say,  any  particular  soil  area  is  seemingly 
an  organic  entity,  functioning  according  to  its  own  inherent 
properties,  but  subject  to  the  modifying  influences  of  environ- 
ment, as  by  exceptional  climatic  extremes,  flood,  fire,  and  especi- 
ally by  artificially  imposed  agencies  of  control. 

From  the  practical  point  of  view  the  problem  of  the  soil  in 
its  relation  to  crop  production  is  like  the  problem  of  the  factory 
or  of  any  other  industrial  endeavor,  in  that  it  is  a  problem  of 
management  or  control.  The  soil  possesses  this  distinction, 
however,  that  it  is  both  the  raw  material  and  the  factory.1  The 
processes  involved  are  physical,  chemical  and  biological,  are 

1  According  to  S.  W.  Johnson — Some  points  of  agricultural  science, 
Am.  Jour.  Sci.  (2),  28,  71-85  (1859) — "The  soil  (speaking  in  the  widest 
sense)  is  then  not  only  the  ultimate  exhaustless  source  of  mineral  (fixed) 
food,  to  vegetation,  but  it  is  the  storehouse  and  conservatory  of  this 
food,  protecting  its  own  resources  from  waste  and  from  too  rapid  use, 
and  converting  the  highly  soluble  matters  of  animal  exuviae  as  well  as  of 
artificial  refuse  (manures)  into  permanent  supplies." 


THE  SOII,  3 

always  numerous  and  interdependent,  and  are  never  (speaking 
generally)  exactly  the  same,  so  that  each  soil  possesses  marked 
individuality.  No  matter  how  soils  may  be  classified,  as  for 
instance  into  provinces,  series  and  types,1  the  fact  remains  that 
the  soil  of  the  individual  field  has  properties  which  give  it  a 
crop-producing  power,  an  adaptation  to  a  specific  crop  or  crop 
rotation,  or  a  responsiveness  to  cultural  treatment,  which  can 
not  be  anticipated  in  any  other  field.  Consequently,  there  is  no 
possibility  of  reducing  soil  management  or  agriculture  to  the 
state  of  an  exact  science.  That  is  to  say,  scientific  investigation 
of  the  problems  involved  cannot  be  expected  to  yield  absolute 
results,  although  furnishing  the  best  possible  basis  on  which  to 
form  judgments.  Soil  management,  like  other  agricultural 
practices,  is  an  art,  more  or  less  well  founded  on  scientific  prin- 
ciples, perhaps,  but  susceptible  of  much  higher  development  as 
the  scientific  principles  involved  become  better  understood. 

1For  definitions,  see  Soil  Survey  Field  Book,  1906,  Bureau  of  Soils, 
U.  S.  Dept.  of  Agriculture,  pp.  15-24.  On  the  ground  that  experience 
has  shown  that  genetic  classifications  are  the  ones  which  have'  generally 
persisted  and  proved  the  most  useful,  objection  might  be  made  to  the 
classification  just  cited.  But  a  careful  inspection  of  the  results  of  the 
Soil  Survey  by  the  U.  S.  Department  of  Agriculture  will  show  that  while 
not  categorically  stating  the  fact,  to  all  intents  and  purposes  it  has 
employed  a  genetic  classification.  This  is  exemplified  by  the  fact  that  its 
delineation  of  soil  provinces  corresponds  quite  closely  with  the  recognized 
physiographic  provinces  of  the  United  States.  See  map  accompanying 
Soils  of  the  United  States,  by  Milton  Whitney,  Bull.  No.  55,  Bureau  of 
Soils,  U.  S.  Dept.  Agriculture,  1909. 


Chapter  II. 

SOIL  MANAGEMENT  OR  CONTROL. 

Aside  from  such  devices  as  greenhouses,  wind-breaks,  etc., 
which  have  a  local  application  only,  there  are  three  general 
methods  of  soil  control:  tillage  methods,  such  as  plowing  and 
harrowing;  rotation  of  crops;  and  the  use  of  soil  amendments 
or  "fertilizers." 

The  existing  knowledge  regarding  tillage  methods  is  generally 
considered  to  be  fairly  satisfactory.  The  purposes  are  well 
understood,  namely,  to  break  up  and  "fine"  the  soil,1  to  keep 
down  weeds,  and  by  forming  mulches  to  decrease  the  loss  of 
water  by  evaporation.  Not  much  increase  is  being  made  in  our 
theoretical  knowledge  of  this  subject,  although  mechanical 
improvements  in  the  implements  of  tillage  are  being  and  will 
undoubtedly  continue  to  be  made. 

The  existing  knowledge  concerning  crop  rotations  is  fairly 
extensive,  but  it  is  almost  entirely  empirical.  Some  at  least  of 
the  purposes  served  by  a  rotation  of  crops  are  fairly  well  known, 
such  as  the  elimination  of  weeds  or  lower  types  of  parasitic 
growth  associated  with  particular  crops;  the  introduction  of 
humus  by  a  grass  crop  or  a  green  manure  crop,  especially  by  the 
Leguminosae  with  their  symbiotic  Azobacteria;  the  improve- 
ment in  the  structure  or  arrangement  of  the  soil  particles  by 
alternating  deep-rooted  and  shallow-rooted  crops;  the  avoidance 
of  continually  growing  a  crop  in  the  presence  of  its  own  excreta, 
products  of  decay,  etc.;  and  lastly,  economic  and  market 
considerations. 

The  existing  knowledge  of  fertilizers,  in  spite  of  a  vast  amount 
of  work  and  an  enormous  literature,  is  still  very  meagre  and  it 
also  is  almost  entirely  empirical ;  and  this  because  studies  on  the 
subject  have  been  dominated  for  three-quarters  of  a  century 
by  one  theory  almost  to  the  exclusion  of  any  other.  The 
exponents  of  this  theory  have  generally  assumed  that  the  action 

1  Actually,  to  granulate  the  soil.  "Fine"  would  seem  to  be  a  mis- 
nomer, but  its  agricultural  significance  is  well  understood,  and  it  has  the 
sanction  of  long  usage  in  the  literature. 


SOIL    MANAGEMENT   OR   CONTROL  5 

of  fertilizers  is  on  the  plant  rather  than  on  the  soil,  and  is 
independent  of  other  factors.  That  is,  while  it  is  admitted  that 
other  factors  influence  plant  growth,  it  has  been  held  that  the 
effect  of  the  fertilizer  is  not  to  modify  the  influence  of  the  other 
factors  but  to  directly  influence  the  plant  by  increasing  its  food 
supply.  As  a  consequence,  it  has  also  been  generally  assumed 
that  the  influence  of  fertilizers  is  additive,  that  is,  the  increase 
in  yield  of  crop  is  proportional  to  the  increase  in  fertilizer  added, 
and  the  increase  in  yield  produced  by  adding  two  fertilizers  is 
the  sum  of  the  increases  which  would  have  been  produced  by 
each  alone.  In  this  form  the  theory  is  essentially  a  quantitative 
one,  and  fertilizer  practice  should  be  easily  susceptible  of  con- 
trol by  chemical  analyses.  But  the  large  mass  of  data  obtained 
from  plot  experiments  shows  that  fertilizer  effects  are  not 
additive.  Indeed,  the  addition  of  some  one  or  more  fertilizer 
constituent  is  sometimes  followed  by  a  decreased  yield.  For 
example,  about  20  per  cent,  of  the  trials  of  fertilizers  on  soils 
growing  corn  and  reported  by  the  American  State  Experiment 
Stations  show  a  decreased  yield.  And  furthermore,  in  spite  of 
the  quantitative  character  of  the  theory,  and  the  numerous 
analyses  of  soils  and  of  plants  which  have  been  made,  there  is  yet 
lacking  any  authoritative  method  for  determining  in  quantitative 
terms  the  fertilizer  needs  of  a  soil.  That  analytical  methods 
have  a  very  restricted  value  in  indicating  even  qualitatively  the 
fertilizer  needs  of  the  soil  is  evidenced  by  the  fact  that  within 
the  past  few  years  a  number  of  the  State  Experiment  Stations 
have  publicly  announced  their  unwillingness  to  undertake  them.1 

*In  this  connection  see:  The  texture  of  the  soil,  by  L.  H.  Bailey, 
Cornell  University  Agr.  Expt.  Sta.,  Bull.  No.  119  (1896);  Suggestions 
regarding  the  examination  of  lands,  by  E.  W.  Hilgard,  University  of 
California,  College  of  Agriculture,  Circ.  No.  25  (1906)  ;  Chemical  analy- 
sis of  soils,  by  William  P.  Brooks,  Massachusetts  Agr.  Expt.  Sta.  Circ. 
No.  11  (1907)  ;  Testing  soils  for  fertilizer  needs,  by  F.  W.  Taylor,  New 
Hampshire  Agr.  Expt.  Sta.,  Circ.  No.  2  (1908)  ;  The  uses  and  limitations 
of  soil  analysis,  by  J.  T.  Willard,  The  Industrialist,  Kansas  State  Agri- 
cultural College,  34,  291  (1908)  ;  Soil  analysis,  by  Wm.  Frear,  Pennsyl- 
vania Agr.  Expt.  Sta.,  Chem.  Circ.  No.  1 ;  How  to  determine  the  fer- 
tilizer requirements  of  Ohio  soils,  by  Chas.  E.  Thorne,  Ohio  Agr.  Expt. 
Sta.,  Circ.  No.  79  (1908)  ;  Concerning  work  which  the  station  can  and 
cannot  undertake  for  residents  of  the  state,  by  Joseph  L.  Hills,  Vermont 
Agr.  Expt.  Sta.,  Circ.  No.  3  (1909). 


6  THE   SOIL   SOLUTION 

The  common  procedure  has  been  to  define  some  arbitrary 
percentage  limit  in  the  soil,  below  which  the  soil  is  supposed  to 
require  fertilizers.  But  the  amount  of  fertilizer  to  be  applied  is 
suggested  on  the  indefinite  basis  of  "experience."  Thus,  Hilgard, 
in  an  interesting  discussion  of  this  subject,1  quotes  Dyer  as 
showing  that  "on  Rothamsted  soils  of  known  productiveness 
or  manurial  condition,  it  appears  that  when  the  citric  acid  extrac- 
tion yields  as  much  as  0.005  Per  cent,  of  potash  and  o.oio  per 
cent,  of  phosphoric  acid,  the  supply  is  adequate  for  normal  crop 
production,  so  that  the  use  of  the  above  substances  as  fertilizers 
would  be,  if  not  ineffective,  at  least  not  a  profitable  investment." 
Hilgard  himself  sets  limits  as  determined  by  strong  hydrochloric 
acid  digestion;  thus  a  soil  containing  upwards  of  0.45  per  cent. 
(K2O)  does  not  need  this  substance  as  a  fertilizer,  while  one 
containing  below  0.25  per  cent,  does  need  it  at  once,  and  inter- 
mediate percentages  indicate  that  potash  fertilizers  would  prob- 
ably be  profitable ;  the  corresponding  upper  and  lower  limits  for 
phosphoric  acid  are  set  at  o.io  per  cent,  and  0.05  per  cent. 
But  Hilgard  points  out  that  various  things,  such  as  the  content 
of  lime,  or  the  texture  of  the  soil,  may  materially  alter  these 
limits.  In  a  very  interesting  set  of  experiments  in  which  white 
mustard  was  grown  in  various  soils,  and  these  same  soils  diluted 
with  various  amounts  of  dune  sand  which  had  previously  been 
extracted  with  strong  hydrochloric  acid,  he  found  that  the  plants 
did  best  when  the  soils  had  been  diluted  with  four  times  their 
weight  of  the  extracted  sand.  This  was  the  case  even  with  a 
pulverulent  sandy  loam ;  and  with  a  black  adobe,  the  best  results 
were  obtained  when  the  diluted  soil  contained  but  0.15  per  cent, 
potash  (K2O)  and  0.04  per  cent,  phosphoric  acid  (P2O5).  It 
also  appears  that  Hilgard  regards  soil  analyses  of  value  only  in 
the  case  of  virgin  soils  or  soils  which  have  been  out  of  cultivation, 
and  in  common  with  other  authorities,  he  fails  to  point  out  how  to 
determine  the  amount  of  fertilizer  needed  by  lands. 

It  is  clear,  therefore,  that  the  principles  underlying  the  practice 
or  art  of  soil  management  and  crop  rotation  are  in  a  state  of 

1  Soils  by  E.  W.  Hilgard,  1906,  p.  339,  et  seq. 


SOII,   MANAGEMENT  OR   CONTROL  7 

development  far  from  satisfactory,  and  scientific  methods  of  soil 
control  yet  wanting.1  Recent  activities  in  soil  investigations, 
however,  justify  the  hope  that  much  improvement  is  to  be 
anticipated,  and  the  application  of  the  modern  methods  of 
physical,  chemical,  and  biological  research  to  the  soil  problem 
promises  a  sure  and  probably  rapid  advance  in  this  branch  of 
applied  science. 

1  It  should,  of  course,  be  borne  in  mind  that  soil  factors  are  not  the 
only  ones  in  crop  production.  Control  by  seed  selection,  breeding  of 
standard  types  of  plants,  etc.,  may  be,  and  probably  is,  more  highly  devel- 
oped than  control  by  soil  factors.  The  same  might  possibly  be  claimed 
for  moisture  supply  in  irrigated  areas ;  but  on  the  other  hand,  such  factors 
as  the  bacterial  and  lower  life  processes  in  the  soil  are  generally  under 
little  or  no  control,  and  as  a  rule  the  amount  and  distribution  of  sunlight 
under  none  at  all.  A  notable  effort  has  been  made  in  the  last  case  with 
shade-grown  tobacco  (see  Bulletins  Nos.  20  and  39,  Bureau  of  Soils, 
U.  S.  Dept.  Agriculture)  and  a  few  cases  are  known  where  shade-crops 
are  employed,  but  not  in  general  agriculture. 


Chapter  III. 

SOU  ANALYSIS  AND  THE  HISTORICAL  METHODS 
OF  SOIL  INVESTIGATION. 

Owing  to  the  labors  of  Davy,  Boussingault,  de  Saussure, 
Liebig,  Sachs,  Knop,  Salm-Horstmar,  and  others  scarcely  less 
distinguished  savants,  it  has  been  clearly  shown  that  growing 
plants  need  certain  mineral  elements  in  order  to  maintain  their 
metabolic  functions,  and  that  these  mineral  elements  can  be  ob- 
tained, under  normal  conditions,  from  the  soil.  All  subsequent 
investigation  has  confirmed  these  statements  and  they  can  now 
be  accepted  as  facts  with  as  much  assurance  as  any  known  law  of 
nature. 

The  determination  and  formulation  of  these  two  fundamental 
facts  came  at  a  time  when  analytical  chemistry  was  being  rapidly 
developed  and  was  finding  wide  and  useful  applications  in  numer- 
ous fields  of  activity.  It  was  natural,  therefore,  that  analytical 
chemistry  should  be  enlisted  in  this  new  field  of  work,  obviously 
of  the  first  importance  to  the  welfare  of  mankind.  It  was  early 
found,  however,  that  the  chemical  analysis  of  a  soil  fails  to  ex- 
plain its  relative  productivity.  In  other  words  the  content  of  a 
soil  with  respect  to  potash,  phosphoric  acid,  or  other  mineral 
plant-food  constituent,  bears  no  necessary  relation  to  its  crop- 
producing  power.  Many  cases  were  found  where  one  soil 
"analyzed  well"  but  did  not  produce  as  large  a  crop  as  another 
soil  which  "analyzed  poor."1  To  meet  this  difficulty  a  subsidiary 
hypothesis  was  brought  forward,  which  rapidly  gained  general 
acceptance  although  lacking  experimental  support. 

This  hypothesis  supposes  that  the  mineral  constituents  of  the 
soil  are  present  in  two  different  chemical  conditions  or  distinct 
kinds  of  combinations,  one  of  which  readily  gives  up  its  con- 
stituents to  growing  plants,  while  the  other  does  not;  and  the 

1  See  also,  Die  Aufnahme  der  Nahrstoffe  aus  dem  Boden  durch  die 
Pflanzen,  von  J.  Konig  und  E.  Haselhoff,  Landw.  Jahrb.,  23,  1009,  1030 
(1894). 


constituents  have,  therefore  been  called  respectively  "available" 
and  "non-available."  It  would  appear  from  his  writings  that 
Liebig  regarded  this  distinction  as  applying  to  the  "absorbed" 
or  "adsorbed"  mineral  matter;  that  is,  on  the  one  hand  the 
material  held  in  or  upon  the  soil  grains  by  surface  forces,  and  on 
the  other  the  chemically  combined  constituents  in  the  minerals 
themselves.  We  know  that  L/iebig  was  much  impressed  by  the 
absorption  experiments  of  Way,  and  himself  did  much  work  in 
this  field.1  But  the  great  body  of  soil  investigators  has  evident- 
ly held  to  the  opinion  that  there  are  two  general  classes  of 
minerals  in  the  soil.  Some  have  held  that  the  "available" 
potassium  is  held  in  zeolites  or  "zeolitic"  minerals,  an  interesting 
example  often  cited  being  glauconite  or  "green  sand  marl," 
which  sometimes  contains  phosphorus  as  well  as  potassium;2 
in  minerals  which  are  easily  broken  down  by  alkaline  solutions, 
as  by  sodium  carbonate  solutions  or  ammonia;  or  in  minerals 
which  are  easily  broken  down  by  organic  acids  supposedly  ex- 
creted from  the  roots  of  growing  plants,  or  formed  by  the  decay 
of  plant  tissue.8 

With  the  advent  of  this  idea  of  a  distinction  between  the 
available  and  non-available  mineral  plant-food  elements  in  the 
soil,  came  attempts  to  distinguish  them  by  analytical  methods. 
Of  these  attempts  we  now  have  a  bewildering  array,  most  of 
1Way  was  misled,  as  we  now  know,  in  considering  the  results  of  his 
absorption  experiments  with  soils  as  merely  metathetical  reactions;  see 
Absorption  by  soils,  by  Harrison  E.  Patten  and  William  H.  Waggaman, 
Bull.  No.  52,  Bureau  of  Soils,  U.  S.  Dept.  Agriculture,  1908. 

2  The  formation  of  zeolites  in  the  soil  has  often  been  assumed,  but 
has  not  yet  been  proven ;  see  Rocks,  rock-weathering  and  soils,  by  George 
P.  Merrill,  1906,  p.  363. 

3  The  classic  experiments  of  Sachs,  in  producing  etchings  on  marble 
slabs,   and  the  etchings  observed  occasionally  on  rock  surfaces  are  the 
proofs  universally  cited.     The  experiments  of   Czapek,  who  substituted 
slabs  of  aluminum  phosphate  and  other  substances  for  the  marble,  and 
those  of  Kossowitch,  show  that  the  action  can  be  accounted  for  more 
satisfactorily  and  reasonably  as  due  to  dissolved  carbon  dioxide.     In  fact 
such  etchings  can  be  produced  on  marble  slabs  by  laying  platinum  wires 
upon  them  and  covering  with  moist   soil,   or   cotton,   or  mats  of  filter- 
paper;  see  Bull.  No.  22,  p.  14,  and  Bull.  No.  30,  p.  41,  Bureau  of  Soils, 
U.  S.  Dept.  Agriculture. 

2 


IO  THE   SOIL   SOLUTION 

them  frankly  empirical.  For  instance,  Hilgard,  in  his  classical 
investigation  of  the  cotton  soils  for  the  Tenth  Census,  treated 
his  soil  samples  with  an  excess  of  hydrochloric  acid,  evaporated 
to  dryness,  extracted  with  water,  and  regarded  the  extracted 
mineral  constituents  as  available.  In  Germany,  a  method  similar 
to  Hilgard's  is  now  in  common  use,  while  in  France  nitric  acid  is 
preferred  generally  because  it  is  supposed  to  have  peculiar  sol- 
vent powers  on  soil  phosphates.  In  the  United  States  the  "offi- 
cial method"  of  the  Association  of  Official  Agricultural  Chemists 
is  to  keep  10  grams  of  the  soil  in  contact  with  100  cc.  of  a  solu- 
tion of  hydrochloric  acid  (specific  gravity  1.115)  at  the  boiling 
point  of  water  for  exactly  10  hours.  In  England  the  popular 
method  is  that  proposed  by  Dyer,  namely,  to  treat  the  soil  with 
a  i  per  cent,  citric  acid  solution,  this  strength  of  solution  being 
supposed  at  one  time  to  represent  the  average  acidity  of  root  sap. 
Maxwell,  in  Hawaii,  and  afterwards  in  Australia,  claimed  good 
results  for  the  extraction  of  the  soil  with  a  I  per  cent,  solution 
of  aspartic  acid,  this  acid  being  employed  on  the  erroneous 
ground  that  the  organic  acids  of  the  soil  are  amido  acids,  and 
that  these  are  the  effective  agents  in  dissolving  the  soil  minerals 
and  rendering  their  constituents  "available."  The  Kentucky 
Agricultural  Experiment  Station  favors  an  N/5  nitric  acid  solu- 
tion,1 but  does  not  recommend  its  use  for  soils  of  other  localities, 
while  in  a  contiguous  state,  the  Tennessee  Station  favors  the 
"official"  method.2  Many  other  methods  have  been  proposed, 
but  the  foregoing  are  typical  and  sufficient  to  illustrate  the  pres- 
ent status  of  soil  analysis. 

It  is  clear  that  these  several  methods  must  give  differing 
results.  And  it  is  not  clear  that  any  one  of  them  is  to  be  pre- 
ferred to  the  others  for  any  reasons  than  analytical  convenience. 
There  is  no  reason  to  expect  that  the  proportion  of  solvent  to 
soil  required  in  these  methods  bears  any  relation  whatever  to 
the  mechanism  of  absorption  by  plant  roots.  And  the  attempts 

1  Soils,  by  A.  M.  Peter  and  S.  D.  Averitt,  Bull.  No.  126,  p.  66  (1906). 
"The  soils  of  Tennessee,  by  Charles  A.  Mooers,  Bull.  No.  78,  p.  49 
(1906). 


SOU,  ANALYSIS  AND  INVESTIGATION  II 

to  simulate  the  properties  of  plant  sap  in  some  of  these  solvents 
are  obviously  illogical,  for  the  plant  sap  does  not  come  in  con- 
tact with  the  soil  grains,  except  through  an  accidental  destruc- 
tion of  the  plant. 

Naturally,  comparisons  were  attempted  between  the  amounts 
of  the  mineral  constituents  extracted  from  a  soil  by  these  vari- 
ous solvents  and  the  amounts  taken  up  by  crops  growing  on  the 
soil.  It  was  found,  however,  that  the  amount  of  any  given 
mineral  constituent  extracted  from  the  soil  by  a  solvent  is  not, 
generally,  the  same  as  that  taken  up  by  the  plant.  Moreover, 
the  ratio  of  one  constituent  to  another  in  the  extract  bears  no 
definite  relation  to  the  ratio  of  these  constituents  in  the  plant. 
Nevertheless  many  efforts  were  made  to  establish  "factors." 
For  instance,  the  percentage  of  potash  extracted  from  the  soil  of 
a  field  by  hydrochloric  acid  is  some  multiple  of  the  percentage 
removed  by  a  wheat  crop ;  it  was  sought  to  determine  this  multi- 
ple, assuming  it  to  be  a  definite  ratio  and  a  natural  constant,  and 
it  was  designated  as  the  potash  factor.  But  there  is  a  different 
factor  for  phosphorus,  another  for  calcium,  and  still  others  for 
each  and  every  constituent.  The  factors  found  for  a  soil  from 
one  area  generally  do  not  hold  for  a  soil  from  another  area. 
Again,  different  factors  obviously  must  be  used  for  different 
crops.  And,  finally,  the  whole  scheme  becomes  hopeless  when  it 
is  realized  that  the  same  crop  will  yield  widely  varying  ash 
analyses,  depending  upon  the  cultural  methods  employed,  the 
judicious  selection  of  seed,  the  amount  and  distribution  of  rain- 
fall and  sunlight,  and  possibly  other  agencies,  all  of  which  affect 
the  growth  and  absorptive  functions  of  the  plant  to  as  great  an 
extent  as  does  the  particular  soil  upon  which  it  may  be  growing. 

Moreover,  from  the  purely  analytical  point  of  view  the  situa- 
tion is  no  better.  For  instance,  the  addition  of  potassium  in  the 
amounts  usually  employed  in  ordinary  fertilizer  practice  general- 
ly does  produce  a  noticeable  effect  on  the  yield  of  crop.  The 
average  application  of  potash  (K2O)  is  certainly  less  than  50 
Ibs.  to  the  acre.  It  is  customary  to  consider  the  surface  foot 
of  soil  as  the  region  affected  by  the  fertilizer,  and  an  acre  foot 
in  good  moisture  condition  weighs  about  4,000,000  Ibs.  To  be 


12 


THE  SOII,  SOLUTION 


conservative,  let  it  be  assumed  that  60  Ibs.  of  potash  have  been 
added  to  3,000,000  Ibs.  of  soil.  The  official  method  of  the 
Association  of  Official  Agricultural  Chemists  calls  for  the  de- 
termination of  the  potash  in  2  grams  of  soil,  which  on  the  basis 
of  the  present  assumption  calls  for  the  estimation  of  an  added 
amount  of  0.00004  gram  potash  or  0.002  per  cent.  Taking 
as  an  example  the  report  of  the  Association  of  Official  Agricul- 
tural Chemists  for  1895*  there  are  given  the  following  results 
obtained  independently  by  a  number  of  analysts,  on  soils  which 
had  presumably  been  sampled  by  the  referee  with  all  possible 


care: 


POTASH  CALCULATED  As  PER  CENT.  OF  THE  FINE  DRIED  EARTH. 


i 

2 

3 

4 

Per  cent. 

Var. 

Per  cent. 

Var. 

Per  cent. 

Var. 

Per  cent. 

Var. 

A  

O  15O 

o  044 

Ct  TZA 

C  OO2 

B  

O  112 

o  180 

Q  

O  ISA 

O  21? 

O  O*7Q 

o  067 

O22C 

O  O7  1 

D  

o  260 

—  O  O"\^ 

E  

o  171 

o  o?8 

O  ITQ 

O  O21 

o  16? 

o  016 

o  T7? 

O  O2  1 

G  

O  OI  I 

O  12? 

O  Oil 

0  286 

O  CtAI 

o  i;8 

o  004 

Mean  .  •  . 

0.315 

CXI56 

0.329 

0.154 

Not  only  do  the  individual  determinations  show  differences 
far  in  excess  of  0.002  per  cent.,  but  the  differences  between  each 
individual  reading  and  the  mean  is  greater  than  0.002  per  cent., 
so  that  it  is  evident  from  these  results  that  the  analytical  pro- 
cedure fails  to  recognize  appreciable  amounts  of  the  so-called 
available  plant  foods.  Consequently  the  "acid  digestion"  of  a 
soil  fails  of  the  purpose  for  which  it  was  designed,  and  it  is  one 
of  the  mysteries  of  chemical  history  that  so  much  time  and  energy 
have  been  devoted  to  such  a  hopeless  quest. 

This  state  of  affairs  is  the  more  surprising  when  the  lim- 
itations of  the  analytical  procedure  are  considered.  The  data 

1  Proceedings  of  the  Twelfth  Annual  Convention  of  the  Association 
of  Official  Agricultural  Chemists,  Bull.  No.  47,  Division  of  Chemistry, 
U.  S.  Dept.  Agriculture,  p.  36  (1896). 


SOIL  ANALYSIS  AND  INVESTIGATION  13 

tabulated  above  indicate  that  the  analyses  were  made  with  an 
exactness  that  justifies  a  statement  to  three  decimal  places,  that 
is,  to  three  significant  figures;  and  in  fact,  as  was  shown,  such 
is  necessary  if  the  figures  are  to  have  any  significance  regarding 
fertilizer  applications.  It  is  obvious  that  the  analysis  of  a  finely 
pulverized  definite  mineral  or  rock  is  less  subject  to  error  than 
a  sample  of  soil  sifted  through  a  2  mm.  mesh.  Yet  the  U.  S. 
Geological  Survey  commonly  reports  its  analytical  data  to  only 
hundredths  of  a  per  cent.,  that  is,  to  two  decimal  places.  What 
variation  may  be  expected  in  duplicate  determinations  by  the 
same  analysts  it  is  difficult  to  say,  for  such  duplicates  are  not 
commonly  published.1  In  spite  of  the  widespread  view  that  the 
chemical  analysis  of  a  soil  is  a  statement  of  great  accuracy,  it 
is  improbable  that  as  usually  determined  the  potash  content  is 
correct  to  three  or  even  two  significant  figures;  it  is  also  doubt- 
ful if  the  phosphoric  acid  content  is  correct  to  even  one  signifi- 
cant figure,  if  the  total  amount  is  below  o.i  per  cent,  of  the  soil. 
That  these  determinations  have  a  higher  accuracy  than  here 
stated  is  not  shown  by  an  inspection  of  the  literature  including 
the  fairly  numerous  results  reported  in  the  annual  Proceedings 
of  the  Association  of  Official  Agricultural  Chemists. 

It  was  early  felt  by  some  investigators  that  soil  analyses  were 
unsatisfactory  for  studying  the  relation  of  the  soil  to  the  food 
requirements  of  a  crop,  and  a  second  method  was  devised,  name- 
ly, the  growing  of  a  crop,  and  determining  the  amount  of 
mineral  constituents  removed  from  the  soil  by  analyzing  the  ash 
of  the  crop.  From  the  point  of  view  of  practical  soil  manage- 
ment this  procedure  involves  the  serious  difficulty  of  being  first 
obliged  to  get  the  crop  before  determining  what  must  be  done  to 
best  get  it.  It  apparently  has  the  scientific  advantage  of  direct- 
ness in  determining  the  mineral  needs  of  the  plant  from  the 
plant  itself.  If  these  needs  were  constant,  the  advantage  would 

1  See :  On  the  interpretation  of  mineral  analyses,  by  S.  L.  Penfield, 
Amer.  Jour.  Sci.,  (4),  10,  33  (1900)  ;  The  analysis  of  silicate  and  car- 
bonate rocks,  by  W.  F.  Hillebrand,  Bull.  No.  305,  U.  S.  Geol.  Surv.,  1907 ; 
Manual  of  the  chemical  analysis  of  rocks,  by  H.  S.  Washington,  1904, 
p.  24;  Ueber  Genauigkeit  von  Gesteinanalysen,  von  M.  Dittrich,  Neues 
Jahrbuch  fur  Mineralogie  und  Palaeontologie,  2,  69  (1903). 


14  THE  SOU,   SOLUTION 

be  real,  but  as  already  mentioned,  one  and  the  same  plant  may 
have  a  very  different  ash  content  as  the  result  of  different  cul- 
tural methods,  different  climatic  and  seasonal  factors,  as  well 
as  different  soils.  Generally,  a  poor  crop  has  a  higher  per- 
centage of  ash  content  than  a  good  crop,  and  sometimes  the 
poor  crop  may  remove  from  the  soil  more  in  absolute  amounts  of 
some  one  or  other  of  the  ash  constituents  than  does  the  good 
crop.  The  ratio  of  the  ash  constituents  is  by  no  means  constant 
for  any  one  crop,  and  of  course  varies  with  different  crops.1 
Finally,  it  is  now  known  that  the  amount  of  the  several  mineral 
nutrients  which  a  soil  must  furnish  to  a  crop  in  the  earlier  stages 
of  growth  is  greater  than  the  crop  contents  at  maturity,2  con- 
sequently an  analysis  of  the  ripe  crop  would  not  indicate  the 
plant's  drain  upon  the  soil  at  all  growing  periods.  So  that, 
while  ash  analyses  have  taught  some  important  things  concern- 
ing plant  growth,  they  have  of  necessity  failed  as  guides  or 
criteria  of  the  crop-producing  power  of  a  soil,  its  fertilizer  re- 
quirements, or  its  content  of  "available"  plant-food. 

A  third  method  of  soil  investigation,  also  essentially  analytical 
in  character,  is  the  plot  or  pot  test.  The  difference  between 
a  plot  or  pot  experiment  is  mainly  one  of  size,  although  it  is 
claimed,  and  with  a  certain  amount  of  justice,  that  the  plot 
experiment  more  nearly  approximates  actual  practice,  and 
should  be  given  a  somewhat  different  consideration  than  the 
more  readily  controlled  pot  experiment.  Here  again  it  has  to 
be  considered  that  seasonal  factors  and  factors  other  than  the 
soil  play  a  relatively  large  part  in  the  production  of  the  crop,  so 
that  conclusions  regarding  the  productivity  of  a  soil  can  not  be 
drawn  from  one  season's  crop.  Also,  nowadays  it  is  recognized 
generally  that  continuous  growing  of  one  crop  is  an  incorrect 

1  For  a  brief  but  comprehensive  discussion  of  ash  analyses  see,  The 
ash  constituents  of  plants,  etc.,  by  B.  Tollens,  Expt.  Sta.  Rec.,  13,  207- 
220,  305-317  (1901-02). 

1  Uber  die  Nahrstoffaufnahme  der  Pflanzen  in  verschiedenen  Zeiten 
ihres  Wachstums,  von  Wilfarth,  Romer  und  Wimmer,  Landw.  Vers.  Sta., 
63,  1-70  (1905)  ;  Plant  food  removed  from  growing  plants  by  rain  or  dew, 
by  J.  A.  Le  Clerc  and  J.  F.  Breazeale,  Year  Book,  U.  S.  Dept.  Agriculture, 
1908,  pp.  389-402. 


SOII,   ANALYSIS   AND   INVESTIGATION  15 

practice,  and  a  rotation  should  be  followed  and  repeated  several 
times  before  conclusions  regarding  the  productivity  of  the  soil 
are  justified.  If,  however,  the  rotation  has  been  well  managed, 
the  cultivation,  fertilizing  and  soil  management  generally  been 
well  done  for  sixteen,  twenty  or  more  years,  the  soil  has  material- 
ly changed,  and  there  can  be  no  assurance  that  the  treatment 
than  best  for  it,  is  that  which  was  best  at  the  beginning  of  the 
experiment.  Therefore  the  method  throws  no  certain  light  on 
the  productive  power  of  the  soil,  or  the  availability  of  its 
mineral  plant-food  constituents.  Although  much  has  been 
learned  from  plot  experiments,  and  especially  from  the  better 
controlled  pot  experiments,  they  are  inadequate  to  meet  the 
fundamental  problem  of  the  relation  of  the  chemical  character- 
istics of  the  soil  to  its  crop-producing  powers. 


Chapter  IV. 

THE  PLANT-FOOD  THEORY  OP  FERTILIZERS. 
The  guiding  principle  in  soil  investigations  for  about  three- 
quarters  of  a  century  and  until  the  past  few  years  has  been  the 
assumption  that  the  principal  function  of  the  soil  is  to  furnish 
mineral  nutrients  to  the  plant,  and  that,  to  supply  a  lack  in  the 
soil,  fertilizers  are  added  because  of  the  mineral  plant  nutrients 
they  contain.  This  theory  has  apparently  much  to  support  it; 
actually,  however,  the  evidence  usually  cited  accords  better  with 
a  more  comprehensive  generalization  which  will  be  formulated  in 
a  later  chapter.  It  is  attractively  simple.  It  will  be  shown 
later,  however,  that  this  very  simplicity  is  an  argument  against 
its  validity. 

Those  substances  which  experience  has  shown  to  be  useful 
soil  amendments  usually  contain  one  or  more  of  the  constituents 
necessary  to  plant  metabolism,  commonly  phosphorus,  potassium, 
nitrogen  or  calcium.  Fertilizers  do  not  always  produce  in- 
creased yields  of  crops,  but  it  has  been  usual  to  consider  bad 
results  as  due  to  other  more  or  less  extraneous  causes.  More- 
over, as  will  appear  later,  crop  yield  is  as  strongly  affected  by 
some  substances  containing  no  mineral  plant  nutrient  as  by 
ordinary  fertilizers.  Again,  the  plant-food  theory  has  been 
apparently  confirmed  by  the  popular  misconception  that  crop 
yields  are  decreasing.  Government  statistics,  however,  indicate 
very  positively  that  crop  yields  are  increasing  in  Europe  as  well 
as  in  America,  more  in  areas  where  the  acreage  is  stationary 
than  in  areas  where  the  acreage  is  increasing,  and  in  areas  where 
fertilizers  are  not  used  as  well  as  in  areas  where  they  are  used. 
Analyses  of  European  soils  which  have  been  cropped  for  cen- 
turies show  no  characteristic  differences  from  the  newer  soils  of 
the  United  States.1  It  is  true  that,  from  bad  management  or 
other  causes,  individual  fields  where  crop  production  has  fallen 

*A  study  of  crop  yields  and  soil  composition  in  relation  to  soil  pro- 
ductivity, by  Milton  Whitney,  Bull.  No.  57,  Bureau  of  Soils,  U.  S.  Dept. 
Agriculture,  1909. 


THE   PLANT-FOOD  THEORY   OF   FERTILIZERS  I/ 

off  are  not  uncommon.  But  that  such  a  condition  is  general 
or  that  it  can  be  associated  generally  with  a  decreased  content 
in  the  soil  of  any  particular  mineral  substance  or  substances,  is 
a  conclusion  not  sustained  by  the  available  data. 

The  plant-food  theory  of  fertilizers  must  now  be  regarded  as 
entirely  insufficient.  Granting  that  it  has  been  useful  in  the 
past  and  has  occasioned  much  valuable  work,  it  seems  to  have 
reached  the  point  which  another  simple  and  temporarily  useful 
theory,  the  phlogiston  theory  of  combustion,  reached  shortly 
before  the  plant-food  theory  of  fertilizers  was  evolved.  Just  as 
the  phlogiston  theory  passed  away  when  the  elementary  nature 
of  oxygen  was  established  and  Lavoisier  taught  the  scientific 
world  to  use  the  balance,  so  the  plant-food  theory  of  fertilizers 
must  pass  with  increasing  knowledge  of  the  relation  of  soil  to 
plant  and  the  application  of  modern  methods  of  research  to  the 
problem. 


Chapter  V. 


THE  DYNAMIC  NATUKE  OF  SOIL  PHENOMENA. 
In  soil  investigations,  until  recently,  the  assumption  has  been 
made,  more  or  less  explicitly,  that  any  given  soil  mass,  as  for 
instance  a  field,  remains  fixed  or  in  place  indefinitely.  It  has 
been  admitted,  of  course,  that  some  physical,  chemical  and 
biological  processes  might  be  taking  place  in  the  soil,  but  these 
have  been  regarded  as  relatively  unimportant  in  their  effects 
upon  the  soil  mass  in  toto.  It  has  been  assumed  that  the  only 
important  change  taking  place  in  the  soil  is  a  loss  of  mineral 
plant  nutrients,  partly  by  leaching,  partly  by  removal  in  the 
garnered  crops.  In  other  words,  the  soil  has  been  regarded  as 
a  static  system.  This  is  a  fundamental  error.  •  In  studying  the 
soil  as  a  medium  for  crop  production,  we  must  consider  the 
plant  itself,  or  at  least  that  part  of  the  plant  which  enters  the 
soil,  namely,  the  root;  the  solid  particles  of  the  soil;  the  soil 
water,  or  the  aqueous  solution  from  which  the  plant  draws  all 
the  materials  for  its  sustenance,  excepting  the  carbon  dioxide 
absorbed  by  its  aerial  portions;  the  soil  atmosphere;  the  biologi- 
cal processes  taking  place.  The  one  common  characteristic  of 
all  these  things  is  that  they  are  continually  in  a  state  of  change; 
therefore  the  soil  problem  is  essentially  dynamic. 

The  root  of  a  growing  plant  is  always  moving.1    The  amount 
of  motion  may  be  small  or  large,  depending  upon  the  surround- 

1  In  order  to  penetrate  the  soil,  a  living  root  must  be  capable  of  exert- 
ing large  pressures,  and  indeed,  the  magnitude  of  these  pressures  has  been 
determined  for  some  cases.  See,  for  citations  of  the  literature,  Pfeffer, 
Plant  Physiology,  translated  by  Ewart,  1903,  Vol.  2,  p.  124  et  seq.  But 
it  cannot  be  doubted  that,  in  general,  root  movement  is  much  facilitated 
and  perhaps  directed  by  movements  among  the  soil  particles.  As  the 
absorbing  tip  of  the  root  removes  film  water  from  the  adjacent  soil  grains, 
there  is  a  necessary  rearrangement  of  these  grains  with  a  shrinking  away 
from  the  tip,  which  then  moves  forward  by  taking  advantage  of  the  move- 
ments among  the  soil  grains. 


THE   DYNAMIC    NATURE   OF   SOIL    PHENOMENA  19 

ing  conditions  or  attendant  circumstances,  but  cessation  of  mo- 
tion means  the  death  of  the  root.  This  becomes  evident  from  a 
consideration  of  the  mechanism  of  root  growth.  The  living 
root  absorbs  and  excretes  water  and  dissolved  substances  through 
a  restricted  area  just  back  of  the  root  tip  or  the  tips  of  the  root 
hairs.  While  absorption  is  taking  place,  however,  there  is  a 
deposition  of  denser  material  over  the  absorbing  area,  or  "root 
corking."  But  coincident  with  the  corking  process,  the  tip  is 
pushed  forward  between  the  soil  grains  into  the  nutrient  medium, 
new  cells  are  formed  and  a  new  absorbing  surface  continually 
brought  into  functional  activity.  A  failure  of  the  plant  root 
to  move  forward  in  this  way  would  mean  a  reabsorption  of  root 
effluvia  with  harmful  consequences  to  the  plant,  or  a  corking  over 
of  the  root  without  further  formation  of  absorbing  surface  and 
with  consequent  cessation  of  its  functioning.  This  would  mean 
the  inevitable  death  of  the  root,  and,  if  general,  of  the  whole 
plant.  It  is  clear,  therefore,  that  root  penetration  and  absorption 
of  plant  nutrients  are  essentially  dynamic. 

The  solid  components  of  the  soil  are  always  in  motion. 
Every  soil,  no  matter  how  flat  the  area  or  how  well  protected 
by  vegetal  covering,  suffers  some  translocation  of  soil  material 
through  rains,  as  is  evidenced  by  suspended  material  in  the  run- 
off waters.  On  hillsides  this  is  shown  by  the  soil  accumulating 
on  the  "up"  sides  of  fences,  especially  stone  fences.  In  the 
aggregate  this  movement  is  probably  quite  large  everywhere. 
It  is  manifestly  so  in  the  watersheds  of  many  of  the  world's  im- 
portant rivers  as  shown  by  their  muddy  waters  and  the  forma- 
tion of  deltas,  sometimes  of  great  area  and  agricultural  im- 
portance. 

With  the  saturation  or  approach  to  saturation  of  the  sur- 
face soil  the  particles  are  more  easily  moved  among  themselves 
by  an  extraneous  force.  It  is  very  rarely  that  the  surface  of  a 
field  is  a  dead  level.  Consequently  when  the  soil  is  wetted,  the 
gravitational  force  on  the  individual  soil  grains  produces  a  more 
or  less  pronounced  "creeping"  effect  down  hill.  On  decided 


2O  THE  SOIL  SOLUTION 

slopes  this  soil  creep  is  believed  to  be  of  great  importance  in 
connection  with  soil  erosion.1 

As  important  as  is  the  translocation  of  material  by  water, 
quite  as  important  probably  is  that  produced  by  the  winds. 
These  are  blowing  all  the  time,  uphill  as  well  as  down,  and 
their  range  of  action  is  thus  far  wider  than  is  that  of  rain  and 
flood.  The  effectiveness  of  the  wind  as  a  translocating  agency 
is  seldom  realized  or  even  suspected  by  the  layman,  although 
it  is  commonly  known  that  the  air  always  contains  some  dust, 
and  dust  storms  are  familiar  phenomena.  That  soil  material 
can  be  carried  long  distances  is  certain,  however,  as  for  instance 
the  sirocco  dust,  often  carried  from  the  Sahara  over  Europe.2 
Dust  carried  high  into  the  air  by  volcanic  eruptions  sometimes 
travels  enormous  distances,  as  in  the  case  of  the  eruption  of 
Krakatoa,  when  such  material  is  reported  to  have  traveled 
thousands  of  miles,  and  volcanic  debris  from  the  eruptions  at 
Soufriere  fell  upon  ships  several  hundred  miles  distant.  Arctic 
explorers  have  reported  the  finding  of  wind-borne  soil  materials 
over  the  polar  ice,  and  mountaineers  have  observed  similar 

1  Soil  erosion  is  undoubtedly  one  of  the  greatest  economic  problems 
of  the  time,  and  yet  there  is  scarcely  any  subject  about  which  there  are 
current  so  many  popular  misconceptions.  In  the  rivers  and  to  those  who 
use  the  rivers  the  water-borne  soil  materials  is  an  unmitigated  nuisance, 
save  possibly  to  a  few  cultivators  of  low-lying  lands  who  for  one  reason 
or  another,  may  flood  their  fields  for  the  sake  of  the  salt  deposited.  To 
the  upland  farmer,  however,  erosion  is  not  only  a  necessity  of  natural 
conditions  which  cannot  be  avoided  entirely,  but  under  proper  control  it 
may  be  even  a  blessing.  The  scalded  and  gullied  hillsides,  a  trial  and 
unnecessary  disgrace  to  the  owner,  are  probably  not  the  main  sources  of 
the  material  which  finds  its  way  to  the  river.  On  the  contrary,  what  are 
regarded  usually  as  well-tilled  fields  supply  the  greater  part  of  the  sus- 
pended material  in  the  rivers.  The  problem  of  erosion  on  the  farm  is 
not  merely  to  check  gullying  and  scalding,  and  deepening  of  stream  heads, 
but  to  so  adjust  both  cropping  system  and  cultural  methods  as  to  secure 
a  reasonable  translocation  of  surface  soil  material  with  a  minimum  con- 
tamination of  the  neighborhood  streams.  See,  Man  and  the  earth,  by 
Nathaniel  Southgate  Shaler,  1905. 

1  For  a  comprehensive  discussion  of  wind  as  a  translocating  agent, 
see:  The  movement  of  soil  material  by  the  wind,  by  E.  E.  Free,  Bureau 
of  Soils,  Bull.  No.  68,  U.  S.  Dept.  Agriculture. 


THE  DYNAMIC   NATURE  OF   SOU,   PHENOMENA  21 

deposits  on  snow-capped  peaks.  Soil  material  on  roofs  and 
similar  inaccessible  places  has  been  observed  many  times,  and 
testifies  to  the  continual  activity  of  the  wind.  The  burial  of 
objects  even  of  considerable  size  by  wind-borne  soil  gives  like 
testimony. 

Measurements  of  the  amount  of  action  of  wind  in  trans- 
locating soil  material  are  rare  and  probably  have  a  qualitative 
value  only.  But  Udden1  in  what  appears  to  be  a  conservative 
calculation,  finds  "the  capacity  of  the  atmosphere  [over  the 
Mississippi  Valley]  to  transport  dust  is  1000  times  as  great  as 
that  of  the  [Mississippi]  River."  The  wind  seldom  is  carrying 
anything  like  so  great  a  load  as  it  is  capable  of  carrying.  That 
is,  the  wind  in  its  attack  upon  the  land  surface  does  not  ordinari- 
ly obtain  so  large  an  amount  of  material  capable  of  being  wind- 
borne  as  it  is  possible  for  the  wind  to  carry  when  suitable  ma- 
terial is  artificially  provided.  It  should  be  remembered  that, 
speaking  generally,  the  velocity  of  the  wind  is  lower  just  at  the 
surface  of  the  ground  than  at  heights  above,  and  it  is  necessary 
to  get  the  soil  material  above  the  surface  before  the  wind  can 
exercise  its  full  efficiency  as  a  carrying  agent. 

Moreover,  wind-borne  material  is  constantly  being  deposited 
as  well  as  being  removed  from  the  land  surface.  It  is  evident, 
however,  that  this  movement  of  soil  material  by  winds  is  very 
great,  and  there  is  no  reason  to  believe  that  it  is  of  any  less 
importance  in  other  areas  than  in  the  Mississippi  Valley.  It  is 
also  evident  that  the  individual  grains  in  any  surface  soil  of  any 
particular  field  or  area  are  continually  and  more  or  less  rapidly 
changing,  and  the  farmer  is  not  dealing  to-day  with  just  the 
same  soil  complex  he  faced  a  few  years  back,  or  will  face  a  few 
years  hence. 

But  besides  the  movements  of  the  solid  components  of  the 
soil  by  translocating  agencies,  other  movements  are  constantly 
taking  place.  Whenever  a  moderately  dry  soil  becomes  wetted, 
it  "swells  up"  until  a  certain  critical  amount  of  moisture  is 
present  above  which  there  is  a  shrinking.  But  as  a  wet  soil 

1  Erosion,  transportation  and  sedimentation  performed  by  the  atmos- 
phere, by  J.  A.  Udden,  Jour.  Geol.,  2,  318-331  (1894). 


22  THE   SOIL   SOLUTION 

dries  out  again  below  the  critical  amount,  there  is  again  a  shrink- 
ing. As  it  is  always  either  raining  or  not  raining,  soils  are  al- 
ways either  getting  wetted  or  are  drying.  Consequently  the 
individual  grains  are  continually  moving  about  among  them- 
selves. A  heavy  object,  such  as  stone,  when  left  on  the  ground 
gradually  sinks  into  it.1  Earthworms,  burrowing  animals  and 
insects  are  continually  at  work  in  most  arable  soils.  The  action 
of  frost  in  "heaving"  a  soil  is  familiar  to  everyone.  Not  so  well 
known,  however,  is  the  fact  that  the  apparently  superficial  cracks 
which  occur  to  a  greater  or  less  extent  in  every  soil,  under 
drought  conditions,  are  in  reality  quite  deep,  extending  well  into 
the  subsoil.  By  the  edges  breaking  off,  and  by  wind-  and  water- 
borne  material  being  carried  in,  considerable  surface  soil  is  thus 
brought  into  the  subsoil.  Through  these  various  agencies, 
therefore,  the  solid  components  of  the  soil  are  continually  sub- 
ject to  much  mixing;  subsoil  is  becoming  surface  soil,  and  to 
some  extent  vice  versa.  An  important  result  of  these  various 
processes  is  the  bringing  into  the  surface  soil  of  degradation  and 
decomposition  products  from  underlying  rocks.  The  processes 
involved  are  essentially  dynamic.2 

The  soil  solution  is  also  a  dynamic  problem.  When  the  rain 
falls  on  the  soil,  a  part,  the  "run-off,"  flows  over  the  surface  and 
finds  its  way  into  the  regional  drainage;  a  part  immediately 
evaporates  into  the  air,  and  is  designated  as  the  "fly-off;"  a 
third  part,  the  "cut-off,"  enters  the  soil.3  The  cut-off  water 
penetrates  the  soil  by  way  of  the  larger  openings  and  interstices, 

1  On  the  small  vertical  movements  of  a  stone  laid  on  the  surface  of 
the  ground,  by  Horace  Darwin,  Proceedings  of  the  Royal  Society  of 
London,  68,  253-261  (1901).  On  the  other  hand,  geological  literature 
would  probably  furnish  numerous  references  to  the  heaving  out  of 
boulders,  probably  as  the  result  of  successive  freezings  and  thawings  of 
the  soil.  The  shape  of  the  stone  as  well  as  the  specific  nature  of  the 
movements  of  the  soil  particles  evidently  has  an  important  influence  in 
determining  whether  the  stone  sinks  into  the  soil  or  vice  versa. 

1  It  is  clear  that  as  the  soil  is  continually  changing  through  physical 
agencies,  the  chemical  analysis  of  it  cannot  be  expected  to  furnish  evi- 
dence as  to  the  mineral  constituents  removed  by  crops  or  by  leaching. 

*  This  terminology  has  been  suggested  by  Dr.  W.  J.  McGee. 


THE   DYNAMIC    NATURE   OF    SOIL   PHENOMENA  23 

and  mainly  under  the  influence  of  gravity.  For  convenience  this 
downward-moving  water  is  designated  as  "gravitational"  water. 
It  moves  through  the  soil  with  comparative  rapidity  and  a  por- 
tion reappears  elsewhere  as  seepage  water,  springs,  etc.  But 
with  the  return  of  fair-weather  conditions  at  the  surface,  there 
is  increased  evaporation  and  augmentation  of  the  fly-off,  and 
there  is  developed  a  drag  or  "capillary  pull"  on  the  water  below. 
A  large  portion  of  the  cut-off  thus  returns  to  the  surface,  main- 
ly through  films  over  the  surface  of  the  soil  grains  and  in  the 
finest  interstices.1 

The  soil  atmosphere  is  continually  in  motion,  following  with 
more  or  less  decided  lag  the  barometric  changes  in  the  atmos- 
phere above  the  soil.  Moreover,  the  chemical  and  physical  pro- 
cesses continually  taking  place  in  the  soil  involve  the  absorption 
or  the  formation  of  free  carbonic  acid,  and  it  seems  probable  that 
all  rain  water  penetrating  the  soil  gives  up  some  oxygen  to  the 
soil  atmosphere.  The  bacteria  and  lower  life  forms  are  nec- 
essarily undergoing  changes  continually.  In  fact  all  compo- 
nents of  the  soil  are  continually  undergoing,  or  are  involved 
in,  changes  of  one  kind  or  another. 

It  is  certain  that  investigation  of  the  various  motions  and 
changes  taking  place  in  the  soil  is  quite  as  important  as  investi- 
gation of  the  soil  components,  and  that  no  clear  idea  of  the 
chemistry  of  the  soil  can  be  obtained  without  it.  The  develop- 
ment of  a  rational  practice  of  soil  control  is  possible  only  when 
the  soil  is  regarded  from  a  dynamic  viewpoint. 

1  Leather,  however,  thinks  the  water  returns  from  only  a  limited  depth, 
some  5-7  feet;  see,  The  loss  of  water  from  soil  during  dry  weather,  by 
J.  Walter  Leather,  Memoirs  of  the  Department  of  Agriculture,  Agricul- 
tural Research  Institute,  Pusa,  India,  Chemical  series,  1,  79-116  (1908). 
Dr.  George  N.  Coffey  has  called  the  author's  attention  to  some  observa- 
tions in  Western  Kansas,  where  a  prolonged  drought  had  dried  the  soil 
to  a  considerable  depth.  A  fairly  heavy  rain  wetted  the  soil  to  less  than 
two  feet  from  the  surface,  and  practically  all  of  this  moisture  had  returned 
to  the  surface  and  evaporated  within  a  few  days.  Such  special  cases  as 
these,  however,  interesting  in  themselves,  are  even  less  so  than  the  normal 
cases  in  humid  areas,  where  a  part  of  the  water  passes  through  the  soil 
as  seepage,  the  larger  portion  returning  to  the  surface,  sometimes  through 
distances  of  many  feet. 


Chapter  VI. 

THE  FILM  WATER. 

When  a  relatively  small  quantity  of  water  is  added  to  an 
absolutely  dry  soil  or  other  powdered  solid,  there  is  some 
shrinkage  in  the  apparent  volume  of  the  soil  or  powder.  The 
water  spreads  over  the  surfaces  of  the  solid  particles  in  a  film, 
and  a  rise  in  temperature  shows  that  a  noticeable  energy  change 
accompanies  the  formation  of  the  film.1  With  further  incre- 
ments of  water  the  apparent  volume  of  the  soil  increases  until  a 
maximum  is  reached.  The  water  content  at  which  this 
maximum  volume  of  soil  can  be  attained  is  a  definite  physical 
characteristic  for  any  given  soil.  What  is  popularly  known  as 
the  "optimum  water  content"  corresponds  to  this  critical  content.2 
It  is  the  point  at  which  further  additions  of  water  will  not 
increase  the  thickness  of  the  moisture  film  on  the  soil  grains,  but 
will  give  free  water  in  the  soil  interstices.  Just  as  the  apparent 
volume  of  a  given  mass  of  soil  varies  with  the  water  content,  and 
reaches  a  maximum  at  a  critical  moisture  content,  so  do  all  the 
physical  properties  vary  and  have  either  a  maximum  or  minimum 
value  at  this  same  critical  moisture  content.  Thus  the  apparent 

1  See,  in  this  connection,  Energy  changes  accompanying  absorption,  by 
Harrison  E.  Patten,  Trans.  Am.  Electrochem.  Soc.,  11,  387-407  (1907)  ; 
see  also  the  recent  valuable  research,  Les  degagements  de  chaleur  qui  so 
produisent  au  contact  de  la  terre  seche  et  de  1'eau,  par  A.  Muntz  et  H. 
Gaudechon,  Ann.  sci.  agron.  (3),  4,  II,  393-443  (1909),  where  it  is  shown 
that  probably  a  part  of  the  heat  is  due  to  chemical  combination  between 
the  water  and  the  other  soil  components.  To  quote,  "Ces  diverses  obser- 
vations nous  conduisent  a  penser,  sans  nous  en  donner  toutefois  la  preuve 
absolute,  que  la  fixation,  de  1'eau  sur  les  elements  terreux  tres  fins  et  sur 
les  materiaux  organises,  est  tout  au  moins,  en  partie,  attribuable  a  une 
combinaison  chimique  qui  se  manifeste  non  seulement  par  un  fort  degage- 
ment  de  chaleur,  mais  aussi  par  la  soustraction  de  1'eau  a  des  substances 
aux-quelles  elle  semble  chimiquement  liee." 

*The  moisture  content  and  physical  condition  of  soils,  by  Frank  K. 
Cameron  and  Francis  E.  Gallagher,  Bull.  No.  50,  Bureau  of  Soils,  U.  S. 
Dept.  of  Agriculture,  1908.  See  also  Uber  physikalische  Bodenunter- 
suchung,  von  H.  Rodewald,  Schriften  Naturwiss.  Vereins  Schleswig- 
Holstein,  14,  397-399  (1909)- 


THE  FILM    WATER  25 

specific  gravity  of  a  soil  reaches  a  minimum,  the  force  required 
to  insert  a  penetrating  tool  becomes  a  minimum,  while  the  rate 
at  which  a  soil  warms  up  reaches  a  maximum,1  and  the  ease 
with  which  aeration  takes  place  reaches  a  maximum.  In  fine, 
this  critical  water  content  is  that  at  which  the  soil  can  be  brought 
into  the  best  possible  physical  condition  for  the  growth  of  crops. 
The  practical  significance  of  the  optimum  water  content  is  far 
greater  than  would  be  supposed  from  the  attention  given  it 
hitherto  by  students  of  the  soil.  It  is  the  content  of  soil  water 
which  the  greenhouse  man  should  strive  to  maintain,  and  which 
the  irrigation  farmer  should  seek  to  provide,  instead  of  the  over- 
wetting  so  common  to  the  practice  of  both.  In  general  farming 
it  is  that  moisture  content  at  which  the  farmer  will  attain  the 
best  results  in  plowing  and  cultivating,  and  attain  these  results 
most  readily. 

With  additions  of  water  beyond  the  critical  point,  there  is  a 
presence  of  free  water  in  the  soil  interstices  accompanied  by 
important  changes  in  the  soil  structure.  With  continued  addi- 
tions, there  is  a  more  or  less  rapid  decrease  in  the  apparent 
volume;  there  is  a  tendency  for  the  soil  aggregates  to  break 
down  and  the  "crumb  structure"  so  greatly  desired  by  agri- 
culturists is  less  and  less  readily  obtained,  and  working  of  the 
soil  tends  in  some  cases  to  produce  that  phenomenon  known  as 
"puddling."  However  desirable  the  property  of  puddling  may 
be  to  the  potter  or  the  brick  maker,  to  the  farmer  it  it  a  bane 
to  be  avoided  above  all  things.  To  overcome  it  requires  his  best 
skill,  and  it  usually  takes  several  years  of  patient  effort  to  restore 
a  puddled  soil  to  good  tilth. 

The  force  with  which  the  film  water  is  held  against  the  soil 
grains  has  not  been  determined  as  yet  with  any  degree  of  pre- 
cision, but  it  is  certainly  very  great.  If  a  soil  be  saturated,  that 
is,  if  so  much  water  be  added  that  further  additions  will  cause  a 
flow  of  free  water,  and  the  soil  be  then  submitted  to  some 
mechanical  device  for  abstracting  the  water,  the  moisture  content 

1  Heat  transference   in   soils,   by   Harrison   E.   Patten,   Bull.    No.   59, 
Bureau  of  Soils,  U.  S.  Dept.  Agriculture,  1909. 
3 


26  THE  SOII,  SOLUTION 

ol  the  soil  can  be  readily  diminished  to  the  critical  water  content ; 
but  to  diminish  it  further  by  mechanical  means  is  not  easy.  The 
tenacity  with  which  film  water  is  held  by  the  soil  grains  has  been 
shown  in  several  ways.  In  one  of  these,  for  instance,  a  semi- 
permeable  membrane  was  precipitated  in  the  walls  of  a  porous 
clay  cell,  which  was  then  filled  with  sugar  solution  having  an 
osmotic  pressure  of  about  35  atmospheres.  When  this  cell  was 
buried  in  a  soil  having  a  moisture  content  above  the  optimum, 
water  flowed  into  the  cell.  On  the  contrary,  when  the  cell  was 
buried  in  another  sample  of  the  same  soil  having  a  moisture  con- 
tent well  below  the  optimum,  there  was  a  marked  flow  of  water 
from  the  cell.  It  would  appear,  therefore,  that  the  attraction 
between  the  soil  grains  and  the  film-forming  water  was  certainly 
greated  than  the  solution  pressure  of  the  sugar.1  Again,  by 
whirling  wetted  soils  in  a  rapidly  revolving  centrifuge,2  fitted 
with  a  filtering  device  in  the  periphery,  and  developing  a  force 
equivalent  on  the  average  to  3,000  times  the  attraction  of  gravita- 
tion, the  soils  could  not  be  reduced  below  the  critical  water  con- 
tent. From  the  results  of  Lagergren,3  Young,4  and  Lord 
Rayleigh,5  it  appears  that  the  force  holding  a  very  thin  moisture 
film  on  the  soil  grains  would  be  of  an  order  of  magnitude  from 
6,000  to  25,000  atmospheres.  This  force,  however,  must  greatly 
decrease  with  thickening  of  the  film,  as  is  shown  by  the  fact 
that  at  the  critical  moisture  content  a  small  further  addition  of 
water  produces  no  marked  heat  manifestation,  though  making 
a  noticeable  difference  in  the  physical  properties  of  the  soil. 

*The  chemistry  of  the  soil  as  related  to  crop  production,  by  Milton 
Whitney  and  Frank  K.  Cameron,  Bull  No.  22,  Bureau  of  Soils,  U.  S. 
Dept  Agriculture,  1903,  p.  54. 

*The  moisture  equivalent  of  soils,  by  Lyman  J.  Briggs  and  John  W. 
McLane,  Bull.  No.  45,  Bureau  of  Soils,  U.  S.  Dept  Agriculture,  1907. 

*Uber  die  beim  Benetzen  fein  verteilter  Korper  auftretende  Warme- 
tonung,  von  Lagergren,  Bihang  till  K.  sv.  Vet-Akad.,  Handl.,  24,  Afd.  II, 
No.  5  (1898). 

*  Hydrostatics  and  elementary  hydrokinetics,  by  George  M.  Minchin, 
p.  311,  1892. 

'On  the  theory  of  surface  forces,  by  Lord  Rayleigh,  PhiL  Mag.  (5), 
30,  285-298,  456-475  (1890). 


THE  FILM   WATER  27 

Therefore,  while  recognizing  that  our  knowledge  of  this  force 
still  lacks  a  desirable  precision,  it  is  nevertheless  clear  that  the 
force  is  very  great 

The  function  of  the  film  water  in  maintaining  the  soil  structure 
is  undoubtedly  important  A  soil  in  good  tilth,  or  good  condition 
for  crop  growth,  shows  a  peculiar  structural  arrangement  of 
the  individual  soil  grains  or  soil  particles,  which  it  is  very  difficult 
to  describe  in  precise  terms,  but  which  is  readily  recognized  in 
practice.  This  condition  is  usually  described  as  a  "crumb  struc- 
ture," either  because  of  its  appearance  or  because  of  the  peculiar 
crumbly  feeling  which  a  soil  in  this  condition  gives  when  rubbed 
between  the  fingers.  The  individual  grains  of  soil  are  gathered 
into  groups  or  floccules.  While  other  causes  may  be  more  or 
less  operative  in  particular  cases,  it  seems  very  probable  that 
the  film  water  is  primarily  the  agency  holding  together  the  grains 
in  these  floccules.  The  obvious  explanation  is  that  the  film  is 
exerting  a  holding  power  because  of  its  surface  tension.  It 
follows,  therefore,  that  anything  which  affects  the  surface  tension 
of  water  should  affect  the  structure  of  the  soil;  that  is,  the 
flocculation  or  granulation  of  the  particles.  But  certain  agents 
which  produce  respectively  flocculation  or  deflocculation,  never- 
theless modify  the  surface  tension  of  the  solution  in  the  same 
direction,  and  in  not  widely  varying  degree.  Similar  difficulties 
arise  in  attempting  to  correlate  "crumbing"  phenomena  with  the 
viscosity  of  the  film  water,1  and  it  must  be  admitted  frankly 
that  present  views  on  this  subject  are  very  unsatisfactory,  and 
that  more  careful  investigation  is  urgently  needed  on  this  funda- 
mental and  important  problem.  Not  only  is  the  absence  of  a 
satisfactory  theory  embarrassing  in  considering  the  problems  of 
soil  structure  and  a  rational  control,  but  the  difficulties  are  no 
less  in  the  equally  important  problems  of  the  movement  of  film 
moisture,  and  the  distribution  of  moisture  in  a  soil. 

1  Equally  unsuccessful  is  the  attempt  to  correlate  flocculating  agents 
with  changes  in  the  density  of  water.  See,  The  condensation  of  water  by 
electrolytes,  by  F.  K.  Cameron  and  W.  O.  Robinson,  Jour.  Phys.  Chem., 
14,  i-n  (1910). 


28 


THE  son,  SOLUTION 


The  movement  of  moisture  into  a  soil  from  an  illimitable 
supply  is  a  comparatively  simple  phenomenon,  controlled  by  a 
rate  law  which  may  be  expressed  by  the  equation  yn  =  kt  when 
y  is  the  distance  through  which  the  movement  has  taken  place; 
t  is  the  time,  and  k  and  n  are  characteristic  constants  for  the 
particular  soil  and  solution.1  This  expression  may  be  more 
readily  recognized  as  a  rate  formula  when  written  dy/at  =•  h.ym, 
where  A  and  m  are  constants  for  the  particular  system.  The 
first  form  of  the  equation  promises  to  be  the  more  useful. 
This  formula  also  describes  the  rate  of  advance  of  a  dissolved 
substance  into  the  soil. 

Owing  to  irregularities  in  the  soil  column  this  equation  is 
more  readily  studied  with  capillary  tubes  or  with  such  absorbents 
as  filter-paper  or  blotting  paper.  The  following  tables  will, 
however,  give  an  idea  as  to  its  validity  for  soils. 

ALLUVIAL  SOIL,  GILA  RIVER. 2 


Time,  '  min. 

Height,  -^  inches 

k  (n  =  1.86) 

2 

1-5 

1.05 

5 

10 

2.4 
3-6 

1.02 
1.  08 

15 
30 
60 

4-3 
6-3 
9.2 

1.  01 
1.05 
1.07 

DISTILLED  WATER  IN  PENN.  LOAM  (/  =  21°  C. ). 


Time,  * 

Height,  y 

k 

Time,  t 

Height,  > 

k 

min. 

cm. 

(n  =  2.25) 

min. 

cm. 

(n  =  2.25) 

I 

I-I5 

1-37 

20 

3-90 

1.07 

2 

1-54 

i-33 

30 

4.67 

1.  06 

3 

1.85 

1-33 

40 

5-39 

I.  II 

4 

2.C8 

1.30 

50 

5-90 

1.09 

5 

2.28 

1.28 

60 

6.47 

1.  12 

7 

2-59 

1.  21 

75 

7.20 

I-I3 

10 

2-97 

1.16 

90 

8.03 

1.  21 

15 

3-47 

1.  10 

105 

8.72 

1-25 

1  See  Bull.  No.  30,  Bureau  of  Soils,  U.  S.  Dept.  Agriculture,  p.  50 
et  seq. ;  also,  The  flow  of  liquids  through  capillary  spaces,  by  J.  M.  Bell 
and  F.  K.  Cameron,  Jour.  Phys.  Chem.,  10,  659  (1906)  ;  see  also,  Wo. 
Ostwald,  2  Supplementheft  Zeitschrift  Kolloidchemie,  1908,  20. 

*  Computed  from  observations  by  Loughridge,  Report  Agr.  Expt.  Sta., 
University  California,  1893-94,  p.  93. 


THE   FILM    WATER 


INDIGO  CARMINE  IN  PENN.  LOAM  SOIL  (/  =  21°  C.). 
Solution  contained  2  grains  dye  per  liter. 


Time,  ' 
min. 

Height,  > 
wet  cm. 

k  for  water 
(«  =  2.25) 

Height  colored 
cm. 

k  for  dye 
(n  =  2.25) 

I 

1.28 

1-75 

0.64 

0-37 

2 

1.67 

1-59 

0.90 

0-39 

3 

2.05 

•1.68 

.. 

.. 

4 

2.26 

1.56 

5 

2.49 

1.56 

I.  O2 

0.21 

7 

2-74 

1.38 

•  • 

•• 

10 

3.20 

1.40 

••     . 

15 

3-72 

1.29 

20 

4.28 

1.32 

1.92 

0.22 

30 

5-io 

I-3I 

40 

5-77 

1.29 

2.69 

0.23 

50 

6.41 

1.26 

3.20 

0.28 

60 

6.90 

1.29 

75 

7.46 

1.23 

90 

8.74 

1.46 

3-59 

O.  2O 

105 

9.00 

1-33 

*  * 

It  has  also  been  shown  repeatedly  by  experiment  that  the 
movement  of  moisture  is  relatively  rapid  when  the  moisture  con- 
tent of  the  soil  is  above  the  optimum,  but  that  the  movement  is 
exceedingly  slow  when  the  soil  has  a  lower  water  content  than 
the  optimum;  that  is,  the  point  at  which  the  water  is  entirely  in 
the  form  of  film  water.  For  instance,  if  a  moderately  wet  sample 
of  soil  be  brought  into  intimate  contact  with  an  air-dry  sample 
of  the  same  soil,  there  will,  at  first,  be  a  relatively  rapid  move- 
ment of  the  moisture,  but  as  soon  as  the  wetted  portion  has  been 
brought  to  the  "optimum"  condition,  no  further  movement  can 
be  detected,  although  the  experiment  has  been  tried  of  leaving 
such  samples  together  for  months  and  with  a  difference  of  water 
content  amounting,  in  the  case  of  clay  soils,  to  15  or  20  per  cent. 
Since  the  drought  limit,  or  the  soil  moisture  content  at  which 
plants  wilt,  is,  for  most  soils,  considerably  below  the  optimum 
water  content,  the  movement  of  film  water  is  obviously  a  problem 
of  the  first  importance  from  a  practical  point  of  view  as  well  as 
of  the  highest  theoretical  interest. 

The  movement  of  water  vapor,  or  its  distillation  from  place 
to  place  in  the  soil,  is  another  problem  often  confused  with  the 
above.  Its  importance  is  not  yet  clear,  although  according  to 


3O  THE  SOII,   SOLUTION 

some  investigators1  it  would  appear  that  the  addition  of  soluble 
fertilizer  salts  by  causing  a  lowering  of  the  vapor  pressure  of 
the  water  induces  a  distillation  to  that  region  from  other  regions 
of  the  soil  as  well  as  from  the  atmosphere  above.  This  brings  up 
the  problem  of  the  diffusion  of  water  and  other  vapors  through 
the  soil.  It  has  been  shown  that  the  soil  "plug"  retards  the 
rate  at  which  diffusion  takes  place  but  induces  no  other  effect 
in  the  ordinary  phenomenon  of  free  diffusion.  This  fact  is 
obviously  of  the  first  importance  in  the  theory  of  mulches,  but 
requires  no  further  consideration  here.2 

1  Sur  la  diffusion  des  engrais  salins  dans  le  terre,  par  Mtmtz  et 
Gaudechon,  Comptes  rendus,  148,  253-258  (1909). 

1  See,  Contribution  to  our  knowledge  of  the  aeration  of  soils,  and 
Studies  of  the  movement  of  soil  moisture,  by  Edgar  Buckingham,  Bulls. 
Nos.  25,  1904,  and  33,  1907,  Bureau  of  Soils,  U.  S.  Dept.  of  Agriculture. 


Chapter   VII. 


THE  MINERAL  CONSTITUENTS  OF  THE  SOU  SOLUTION.1 

The  mineral  constituents  of  the  soil  are  products  of  the  dis- 
integration, degradation  and  decomposition  of  rocks.  The  de- 
composition products  are  mainly  silica  in  the  form  of  quartz, 
ferruginous  material  consisting  of  more  or  less  hydrated  ferric 
oxide  and  alumina,  and  hydrated  aluminum  silicate.  The 
ferruginous  material,  being  deposited  or  formed  in  the  soil  in  a 
very  finely  divided  condition,  frequently  coats  the  soil  frag- 
ments to  such  an  extent  as  completely  to  mask  their  true  char- 
acter. But  if  a  soil  be  thoroughly  shaken  with  water,  and 
especially  in  the  presence  of  some  deflocculating  agent  such  as  a 
slight  excess  of  ammonia,  as  in  the  ordinary  preparation  of  a 
soil  sample  for  mechanical  analysis2  the  coating  material  is 
generally  removed  quite  readily,  and  the  mineral  particles  appear 
as  fragments  and  splinters  of  the  ordinary  rock- forming 
minerals.  Sometimes  these  fragments  are  more  or  less  worn 
and  rounded  at  the  edges,  showing  mechanical  abrasion  or 
solvent  action;  sometimes  they  show  evidences  of  partial  alter- 
ation and  decomposition;  but  surfaces  of  the  unaltered  mineral 
individuals  always  are  found.  These  unaltered  minerals  occur 
as  fragments  of  all  sizes,  and  are  to  be  found  in  the  sands,  silts, 
and  presumably  in  the  clays.  As  might  be  anticipated,  the 
minerals  other  than  quartz  generally  show  a  tendency  to  segre- 
gate in  the  finer  mechanical  separations  of  the  soil.  The  presence 
of  these  unaltered  mineral  fragments  in  the  clays  has  so  far 
defied  direct  experimental  proof  because  of  the  limitations  of 
the  microscope,  but  from  chemical  reasoning  and  a  priori  con- 

1  For  a  more  detailed  discussion  and  citations  of  the  literature,  see 
The  mineral  constituents  of  the  soil  solution,  by  Frank  K.  Cameron 
and  James  M.  Bell,  Bull.  No.  30,  Bureau  of  Soils,  U.  S.  Dept.  Agricul- 
ture, 1905. 

1  Centrifugal  methods  of  mechanical  soil  analysis,  by  L,.  J.  Briggs, 
F.  O.  Martin  and  J.  R.  Pearce,  Bull.  No.  24,  Bureau  of  Soils,  U.  S.  Dept. 
Agriculture,  1904. 


32  THE   SOIL   SOLUTION 

siderations  there  can  be  but  little  doubt  that  they  exist  in  the 
clays  as  in  the  coarser  separations.1 

The  minerals  to  be  anticipated  in  the  soil  are  those  commonly 
occurring  in  the  rocks;  but  as  a  result  of  the  action  of  mixing 
and  transporting  agencies,  a  soil  normally  contains  minerals  from 
rocks  other  than  those  from  which  it  is  primarily  derived. 

It  would  hardly  be  fair  to  regard  a  beach  sand,  for  instance,  as 
a  normal  soil.  Yet  it  is  surprising  how  many  minerals  other 
than  quartz  can  usually  be  found  even  in  a  beach  sand. 
Opinions  may  differ  as  to  just  what  are  the  common  rock- form- 
ing minerals,  and  perhaps  no  two  mineralogists  or  petrographers 
would  give  identical  lists,  but  there  are  a  number  of  minerals 
which  would  appear  undoubtedly  in  every  list,  and  these  would 
be  found  generally  in  any  soil.  Again,  it  might  happen  that  in 
any  given  sample  of  soil,  no  pyroxene,  for  instance,  could  be 
found;  but  experience  shows  that  it  would  never  happen  in  such 
a  case  that  no  amphibole,  chlorite,  serpentine,  or  other  ferro- 
magnesian  silicates  would  be  present.  However  distinct  these 
minerals  cited  may  be  from  each  other  morphologically  or 
optically,  they  are  much  the  same  in  their  chemical  character- 
istics, their  solubilities  and  their  reactions  with  water  and  such 
dilute  solutions  as  exist  in  the  soil.  Hence  from  the  point  of 
view  of  the  soil  chemist  they  may  be  considered  for  all  practical 
purposes  varieties  of  one  and  the  same  mineral  species.  Con- 
sequently an  important  result  of  researches  on  the  minerals  of 
the  soil  is  the  generalization  that  soils  are  far  more  heterogeneous 
than  are  rocks,  and  that  practically  every  soil  contains  all  the 
common  rock-forming  minerals.3 

It  is  not  difficult  to  account  for  the  heterogeneity  of  the  mineral 
content  of  the  soil.  Many  of  our  rocks  are  reconsolidated  soils, 

1  See,  The  mineral  composition  of  soil  particles,  by  G.  H.  Failyer, 
J.  G.  Smith  and  H.  R.  Wade,  Bull.  No.  54,  Bureau  of  Soils,  U.  S.  Dept. 
Agriculture,  1909.  Recent  improvements  in  microscope  methods  make  it 
possible  to  identify  without  serious  trouble  the  mineral  content  of  silts 
with  a  diameter  as  low  as  0.005  mm.,  and  many  even  of  the  clay,  particles 
have  recently  been  determined  with  satisfactory  accuracy. 

*  See  Bull.  No.  30,  Bureau  of  Soils,  U.  S.  Dept.  Agriculture,  1905, 
p.  9. 


THE   MINERAL   CONSTITUENTS   OF   THE)   SOIL   SOLUTION         33 

and  the  alternating  formation  of  rock  and  soil  from  the  same 
materials  is  probably  an  agency,  in  some  part  at  least,  in  the 
mixing  of  soil  material.  The  action  of  water  in  carrying  off 
and  transporting  surface  material  and  in  gullying  and  eroding 
sloping  surfaces  is  probably  a  large  factor.  But  this  agency, 
like  the  first,  is  rather  restricted  and  localized.  Just  as  important 
as  a  mixing  agency  is  the  wind.  This,  unlike  water,  works  up- 
hill as  well  as  down,  and  is  more  or  less  in  action  at  all  times, 
continually  transporting  soil  material  from  place  to  place.  Wind- 
borne  dust  on  roofs  of  dwellings,  on  rocky  mountain  tops  and 
similar  places,  where  it  could  have  been  brought  by  no  other 
agency  than  the  wind,  is  sometimes  found  supporting  vegetation. 
Many  chemical  and  mineralogical  analyses  of  wind-borne  dust 
obtained  from  various  locations  show  it  to  have  generally  the 
same  essential  characteristics  as  ordinary  soils. 

Aside  from  the  quartz  and  ferruginous  materials  mentioned 
above,  the  major  part  of  the  soil  minerals  are  silicates,  ferro- 
silicates,  alumino-silicates  or  ferro-alumino-silicates,  of  the  com- 
mon bases,  sodium,  potassium,  calcium,  magnesium,  and  ferrous 
iron.  Other  bases,  such  as  lithium,  barium,  or  the  heavy  metals 
may  occasionally  be  present  in  appreciable  amounts  as  may 
other  types  of  silicates,  or  other  mineral  salts,  but  these  may  be 
regarded  as  more  or  less  incidental  and  rarely  affecting  in  any 
essential  way  the  general  character  of  the  soil  mass.  These 
silicates  or  silico  minerals  are  all  somewhat  soluble  in  water, 
and  being  salts  of  weak  acids  with  strong  bases,  are  greatly 
hydrolyzed.  A  convenient  illustration  is  afforded  by  the  well- 
known  rock  and  soil  mineral,  orthoclase.  Assuming  its  type 
formula,  the  reaction  with  water  may  be  represented. 
K.AlSi3O8  +  HOH  ±5  H.AlSi3O8  +  KOH. 

Under  ordinary  soil  conditions,  with  a  relatively  large  propor- 
tion of  carbon  dioxide  in  the  soil  atmosphere,  the  potash  formed 
would  be  more  or  less  completely  transformed  to  the  bicarbonate, 

KOH  -f  CO2  +  H2O  ±*  KHCO3  +  H2O. 

Confirmation  of  this  view  is  afforded  by  the  natural  associations 
and  known  alteration  products  of  orthoclase. 


34  THE  SOU,   SOLUTION 

The  acid  of  the  formula  H.AlSi3O8  is  not  known  and  is 
probably  entirely  instable  under  ordinary  conditions,  and  breaks 
down  with  the  separation  of  silica,  to  form  the  minerals  pyro- 
phyllite,  kaolinite  or  kaolin,  and  diaspore  according  to  the  follow- 
ing equations : 

H.AlSi3O8  — SiO2  =  H.AlSi2O6  (Pyrophyllite) 
H.AlSi3O8  — 2SiO2  =  H.AlSiO4  (Kaolinite) 
H.AlSi3O8  --  3SiO2  =  H.A1O2  (Diaspore). 

All  three  of  these  minerals  and  their  corresponding  salts  have 
been  found  in  nature  as  alteration  products  of  orthoclase.  It 
is  probable  that,  under  soil  conditions,  the  principal  metamorphic 
product  of  feldspar  is  kaolin  (or  kaolinite  when  it  is  crystalline), 
hydrated  aluminum  oxide  being  of  much  less  importance1  and 
pyrophyllite  of  doubtful  occurrence.  A  still  more  interesting 
case,  perhaps,  because  of  the  well  recognized  tendency  of  mag- 
nesium salts  to  form  basic  compounds,  is  the  alteration  of 
pyroxene,  amphibole  and  olivine  with  the  formation  of  a  chlorite 
or  serpentine,  common  associations  in  nature,  which  may  be 
represented 

MgSi03  -f  HOH  ±5  MgSiO3.wMg(OH)2  -f  SiO2. 

It  is  tacitly  assumed  in  the  foregoing  statements  that  the 
reaction  between  a  silicate  mineral  and  water  is  a  reversible 
reaction.  This  is  not  definitely  known  to  be  the  case,  for  the 
formation  of  the  ordinary  silicate  rock-forming  minerals  in  the 
wet  way  at  ordinary  temperatures  has  as  yet  been  realized  in 
only  a  few  cases.  The  assumption  has,  however,  some  experi- 
mental support.  Minerals  have  been  often  made  in  the  wet  way 
at  somewhat  elevated  temperatures,  especially  interesting  cases  in 
this  connection  being  the  formation  of  orthoclase  by  Friedel  and 
Sarasin2  at  slightly  elevated  temperatures,  and  the  formation  of 

1  See  Ueber  die  Bildung  von  Bauxit  und  verwandte  Mineralien,  von 
A.  Liebrich,  Zeit.  prakt.  Geol.,  1897,  212-214. 

*  Sur   la   reproduction   par   voie   aqueuse   du    feldspath   orthose,   par 
Friedel  et  Sarasin,  Comptes  rendus,  92,  1374  (1881). 


THE   MINERAL   CONSTITUENTS   OF  THE  SOIL,   SOLUTION         35 

zeolites  by  Gonnard1  and  by  Doroshevskii  and  Bardt,2  and  the 
formation  of  apatite  by  Weinschenk.3  Feldspars  and  zeolites  are 
common  natural  associations,  it  being  generally  conceded  that 
zeolites  are  alteration  products  of  the  feldspars  through  the 
action  of  water;  but  Van  Hise4  has  pointed  out  that  under  con- 
ditions of  weathering  such  as  would  obtain  in  the  soil,  the  tend- 
ency is  for  the  zeolites  to  alter  to  feldspars.  Wohler's  classical 
experiment  of  recrystallizing  apophyllite  from  hot  water5  is 
significant,  for  only  the  products  of  hydrolysis  should  be  obtained 
if  there  is  an  irreversible  reaction  between  the  mineral  and 
water.  Lemberg  found  that  leucite  (KAlSi2O6)  when  treated 
with  an  aqueous  solution  containing  10  per  cent,  or  more  of 
sodium  chloride,  was  partially  transformed  to  analcite 
(NaAlSi2O6.wH2O),  potassium  chloride  being  formed  at  the 
same  time.  The  reverse  reaction  was  also  realized,  that  is,  the 
partial  conversion  of  analcite  to  leucite  by  treatment  with  a 
solution  of  potassium  chloride,  and  similar  transformations  were 
carried  out  with  the  feldspars.6  L,emberg's  experiments  are  of 
especial  value  as  they  were  carried  out  at  ordinary  as  well  as  at 
high  temperatures.  It  appears  probable,  therefore,  that  the 
hydrolysis  of  a  silicate  of  the  alkalis  or  alkaline  earths  is  a  revers- 
ible reaction.  It  should  be  noted,  however,  that  Kahlenberg  and 
Lincoln7  have  shown  that  probably,  in  very  dilute  solutions  of 

*Note  sur  une  observation  de  Fournet,  concernant  la  production  des 
zeolites  a  froid,  par  F.  Gonnard,  Bull.  Soc.  min.  France,  5,  267-269  (1882)  ; 
Jahrb.  Min.,  1884,  I,  Ref.  28. 

*  Metathetical  reactions  with  artificial  zeolites,  by  A.  Doroshevskii 
and  A.  Bardt,  Jour.  Russ.  Phys.  Chem.  Soc.,  42,  435-42  (1910)  ;  Chem. 
Zentr.,  1910,  II,  68. 

8  Beitrage  zur  Mineralsynthesis,  von  E.  Weinschenk,  Zeit.  Kryst.,  17, 
489-504  (1890). 

4  U.  S.  Geol.  Surv.  Monograph,  47,  A  treatise  on  metamorphism,  by 
Charles  R.  Van  Hise,  1904,  p.  333. 

5Jahresb.  Fortschr.  Chemie  Liebig  and  Kopp,  1847-48,  1262;  note. 

8  Ueber  Silicatumwandlungen,  von  J.  Lemberg,  Zeit.  deutsch.  geol. 
Ges.,  28,  519-621  (1876)  ;  Inaug.  diss.  Dorpat,  1877;  Bied.  Centbl.,  8,  567- 
577  (1879). 

T  Solutions  of  silicates  of  the  alkalis,  by  L.  Kahlenberg  and  A.  T. 
Lincoln,  Jour.  Phys.  Chem.,  2,  77-90  (1898). 


36  THE  SOIL  SOLUTION 

alkali  silicates,  the  hydrolysis  is  practically  complete  and  the 
silica  is  nearly  all  present  as  colloidal  silica  and  not  as  silicic 
acid.  Nevertheless  at  higher  concentrations  silicates  are  formed, 
and  there  is  abundant  evidence  in  nature  that  the  alumino-  or 
ferro-silicates  are  reacting  with  bases  to  form  salts,  for  example 
such  as  the  micas.1  If  the  hydrolysis  were  quite  complete,  it 
would  appear  to  follow  that  the  reaction  between  water  and  the 
silicate  is  irreversible.  In  that  case  it  is  difficult  to  see  how  any 
silicate  mineral  could  persist  in  the  soil  for  any  length  of  time, 
and  all  soils  should  soon  become  sterile  wastes  composed  essen- 
tially of  quartz,  kaolin  and  ferruginous  oxides.  It  has  been 
suggested  that  the  original  mineral  particles  are  protected  from 
decomposition  by  the  formation  of  a  coating  "gel."  That  is, 
that  silica,  alumina,  ferruginous  or  other  materials  result  from 
the  decomposition  of  the  minerals  in  a  jelly-like  form  on  the  sur- 
face of  the  soil  grains,  protecting  them  from  further  action  of 
the  soil  solution.2  If  diffusion  can  take  place  through  the  gel, 
solution  and  hydrolysis  of  the  mineral  would  proceed,  although 
the  presence  of  the  gel  would  probably  retard  the  rate  of  the 
reaction.  If  it  be  postulated,  however,  that  diffusion  through 
the  gel  does  not  take  place,  the  minerals  of  the  soil  can  have  no 
influence  on  the  composition  of  the  soil  solution,  which  is  an 
unthinkable  alternative.  The  presence  of  such  gels  in  the  soil 
has  frequently  been  assumed,  but  satisfactory  proof  is  generally 
wanting. 

In  general,  the  same  kind  of  considerations  developed  for 
orthoclase  hold  for  the  other  soil  minerals.  If  minerals  of  this 
character  be  pulverized  or  ground  reasonably  fine  and  then  be 

1  Van  Hise,  loc.  cit.,  p.  693. 

2  A  gel  is  a  jelly-like  substance,  apparently  continuous,  which  forms 
either  by  the  settling  from  suspension  in  a  liquid  of  very  fine  particles 
which  then  become  aggregated ;  or,  is   formed  by  the  evaporation  of  a 
liquid  containing  fine  particles  in  suspension  until  the  quantity  of  liquid 
remaining  is  just  sufficient  to  serve  as  a  cementation  medium  holding  the 
suspended  particles  together  in  a  semi-rigid  mass.     For  an  experimental 
demonstration  of  the  formation  of  such  a  gel,  see,  The  effect  of  water 
on   rock  powders,   by  Allerton   S.    Cushman,    Bull.    No.   92,    Bureau   of 
Chemistry,  U.  S.  Dept.  Agriculture,  1905. 


THE   MINERAL   CONSTITUENTS   OF  THE   SOIL   SOLUTION         37 

shaken  with  distilled  water  which  has  been  previously  boiled 
to  eliminate  the  dissolved  carbon  dioxide,  the  resulting  solution 
will  give  an  alkaline  reaction  with  such  indicators  as  phenol- 
phthalein  or  litmus.1  If  a  soil  be  shaken  up  thoroughly  with 
water,  the  resulting  solution  filtered  free  of  suspended  matter, 
as  by  passing  through  a  Pasteur-Chamberland  bougie,  and  then 
boiled  to  eliminate  the  carbon  dioxide,  in  the  vast  majority  of 
cases  the  solution  will  also  give  an  alkaline  reaction  with  phenol- 
phthalein  or  litmus.  The  waters  of  most  of  our  springs,  ponds, 
creeks  or  rivers  being  natural  soil  solutions,  give  an  alkaline 
reaction  after  boiling. 

But  the  mineral  content  of  these  natural  waters  varies  greatly. 
These  waters  are  composed  in  part  of  the  "run-off,"  in  part  of 
a  portion  of  the  "cut-off"  waters,  described  above.  This  por- 
tion of  the  cut-off,  normally,  in  passing  through  the  soil  goes 
mainly  through  the  larger  interstices.  It  is  not  long  in  contact 
with  the  individual  soil  particles  and  floccules,  and  because 
diffusion  of  dissolved  mineral  substances  is  quite  slow,  especially 
in  dilute  solutions,  it  takes  up  but  little  mineral  matter  from 
such  aqueous  films  as  it  may  intercept. 

A  different  state  of  things  exists  with  that  portion  of  the  cut- 
off water  which  returns  towards  the  surface  by  reason  of  capil- 
lary forces,  to  form  the  great  natural  nutrient  medium  for  plants. 
This  water  is  moving  over  the  soil  particles  in  films,  and  with 
slowness.  It  is  long  in  contact  with  successive  fragments  of 
any  particular  mineral  and  all  the  different  minerals  making 
up  the  soil.  Consequently,  it  tends  towards  a  saturated  solution 
with  respect  to  the  mineral  mass;  and  it  follows  that  if  every 
soil  contains  all  the  common  rock-forming  minerals,  every  soil 
should  give  the  same  saturated  solution,  barring  the  presence  of 

1  In  making  such  experiments  in  the  laboratory  or  in  lecture  demon- 
strations, it  is  well  to  have  the  mass  of  water  large  in  comparison  with 
the  mass  of  powdered  mineral  or  rock;  otherwise  secondary  adsorption 
effects  may  occur  and  obscure  the  results  of  the  hydrolysis. 

57850 


38  THE  SOIL  SOLUTION 

disturbing  factors.1  Disturbing  factors,  however,  enter  into  all 
cases  under  field  conditions,  such  for  instance  as  the  presence 
of  some  uncommon  or  unusual  mineral  in  appreciable  amounts, 
differences  in  temperature,  surface  effects,  or  extraneous  sub- 
stances. These  will  be  considered  later,  but  another  disturbing 
factor  requires  immediate  consideration. 

In  every  soil,  varying  proportions  of  the  soluble  mineral  con- 
stituents are  present  otherwise  than  as  definite  mineral  species; 
that  is,  they  are  present  as  solid  solutions,  or  absorbed  on  the 
soil  grains  or  perhaps  absorbed  in  some  other  manner.  The  con- 
centration of  the  liquid  solution  in  contact  with  a  solid  solution 
or  complex  of  absorbent  and  absorbed  material  is  dependent 
upon  the  relative  masses  of  solution  and  solid.  Thus,  the  con- 
centration of  a  solution  with  respect  to  phosphoric  acid,  when 
brought  into  contact  with  so-called  basic  phosphates  of  lime  or 
iron,  is  dependent  in  a  marked  way  upon  the  proportion  of  solu- 
tion to  solid.2  Consequently  it  is  to  be  expected  that  an  aqueous 
extract  of  a  soil  will  vary  in  concentration  with  the  proportion 
of  water  used;  and  that  with  the  same  proportion  of  water, 
different  soils  or  different  samples  of  the  same  soil  will  yield 
different  concentrations. 

How  far  absorbed  mineral  constituents  affect  the  solubility 
of  the  definite  minerals  in  the  soil  or  influence  the  concentration 
of  the  soil  solution,  it  is  not  possible  to  predict  with  any  approach 
to  certainty.  Those  soils  which  hold  the  most  moisture  are 
generally  the  best  absorbers.  Moreover,  the  soluble  mineral 
constituents  of  the  soil,  for  instance  potassium  or  phosphoric 
acid,  are  absorbed  to  a  very  high  degree  from  dilute  solutions. 

1  Feldspars  certainly,  and  phosphorites  possibly,  are  mineral  compo- 
nents of  the  soil;  and  these  substances  when  ground  sufficiently  fine  have 
been  added  to  soils  with  sometimes  an  increased  production  of  crop.    Other 
minerals,  such  as  leucite,  have  given  similar  results.     But  also  apparently 
pure  quartz  sand  sometimes  accomplishes  the  same  results,  as  for  example, 
in   the   experiments   of   Hilgard   cited   above.     It  has   not   been   shown, 
however,  that  the  addition  of  any  of  these  substances  produces  an  appre- 
ciable change  in  the  concentration  of  the  soil  solution. 

2  The  action  of  water  and  aqueous  solutions  upon  soil  phosphates,  by 
Frank  K.   Cameron  and  James  M.  Bell,  Bull.  No.  41,  Bureau  of  Soils, 
U.  S.  Dept.  of  Agriculture,  1907. 


THE   MINERAL   CONSTITUENTS  OF  THE  SOIL   SOLUTION         39 

Consequently  it  is  to  be  expected  that  variations  in  the  concen- 
tration of  the  natural  soil  solution  would  be  less  than  in  aqueous 
extracts,  when  there  is  employed  a  constant  and  relatively  large 
proportion  of  water  to  soil.  These  considerations  are  of  great 
theoretical  importance  since  they  appear  to  negative  the  possi- 
bility of  getting,  with  present  experimental  resources,  any  exact 
knowledge  of  the  concentrations  of  the  mineral  constituents  in 
the  soil  solution  when  the  soil  is  in  condition  to  grow  the  com- 
mon crop  plants.  Moreover,  they  furnish  a  guide  to  the  limi- 
tations which  must  be  recognized  in  attempting  to  postulate 
what  these  concentrations  may  be  on  the  basis  of  analytical  data 
obtained  from  aqueous  soil  extracts. 

Many  attempts  have  been  made  to  extract  the  solution  natur- 
ally existing  in  the  soil  and  to  analyze  it.  The  results  obtained 
have  not  been  very  satisfactory,  owing  mainly  to  the  mechanical 
difficulties  involved.  As  pointed  out  above,  the  solution  in  a 
soil  under  suitable  conditions  for  crop  growth  is  held  by  a  force 
of  great  magnitude.  Nevertheless,  by  using  powerful  centri- 
fuges, with  saturated  soil,  it  has  been  possible  to  throw  out  the 
excess  of  solution  over  the  critical  water  content  of  the  soil. 
In  this  way  small  quantities,  generally  a  very  few  cubic  centi- 
meters at  a  time,  have  been  obtained.  The  analysis  of  a  few  cubic 
centimeters  of  a  very  dilute  solution  is  in  itself  difficult,  involv- 
ing necessarily  more  or  less  uncertainty  as  to  the  absolute  value 
of  the  results.  Nevertheless,  the  concentration  of  the  soil  solu- 
tions thus  obtained,  with  respect  to  phosphoric  acid  and  potash, 
varied  but  little  for  soils  of  various  textures  from  sands  to 
clays,  and  the  variations  observed  could  not  be  correlated  with 
the  known  crop-producing  power  of  the  soils.  The  average 
concentrations  of  the  soil  solutions  thus  obtained  lies  in  the 
neighborhood  of  6-8  parts  per  million  (p.p.m.)  of  solution  for 
phosphoric  acid  (P2O5)  and  25-30  parts  per  million  for  potash 
(K.,0).1  In  the  following  table  are  given  the  results  obtained 
1  In  this  connection  it  is  interesting  to  note  that  recent  investigations 
on  the  proportions  of  phosphoric  acid,  potassium  and  nitrates  in  cultural 
solutions  best  adapted  to  the  growth  of  wheat,  give  the  same  ratio  of 
phosphoric  acid  to  potassium  as  the  figures  just  cited  show  to  exist  nor- 
mally in  the  soil  solution. 


THE  SOIL  SOLUTION 


by  analyzing  solutions  extracted  from  different  samples  of  loams 
and  sands  by  means  of  a  centrifuge.  The  crop  growing  on  these 
soils  and  the  crop  condition  at  the  time  the  samples  were  col- 
lected are  given  in  the  table,  and  the  percentages  of  water  in 
the  samples  when  placed  in  the  centrifuge  are  also  given. 

ANALYSIS  OF  SOIL  SOLUTION  REMOVED  FROM  FRESH  SOILS 
BY  THE  CENTRIFUGE. 


Soil 

Crop 

Condition 
of  crop 

Per  cent, 
moisture 

Parts  per  million 
of  solution 

P04 

Ca 

K 

L/eonardtown  loani  

Wheat 
Wheat 
Wheat 
Clover 
Corn 
Corn 
Wheat 
Wheat 
Corn 
Forest 
Corn 
Wheat 
Wheat 
Corn 

Good 
Poor 
Good 
Good 
Medium 
Medium 
Good 
Poor 
Good 
Poor 
Good 
Good 
Poor 
Medium 

22.0 
25.2 
I7.6 
19.7 
17-5 
I8.3 

18.8 

2O.O 

17-3 
IO.O 

11.9 

10.7 

II.  2 

10.6 

6 

10 

8 

5 
8 
8 
7 
7 
8 

5 
ii 
18 
8 
9 

17 
9 

22 

18 

8 

44 
27 
24 
18 

36 
45 
38 
65 

22 

*9 
38 
19 
36 
25 

34 
24 
25 
3i 
3i 
3i 
24 
35 

L/eoiiariltowii  loam  

Sassafras  loam           

The  concentrations  of  the  solutions  obtained  from  the  samples 
do  not  justify  any  correlation  with  the  crop-producing  power 
of  the  soils,  nor  with  the  texture  of  the  soils.  The  wide  varia- 
tion in  the  concentrations  with  respect  to  calcium  is  probably 
due  to  the  fact  that  all  of  the  samples  came  from  fields  which 
had  been  limed,  some  quite  recently,  and  that  the  content  of 
carbon  dixoide  in  the  different  samples  varied.  It  is  of  special 
interest  to  note  that  the  content  of  calcium  in  the  solutions  does 
not  show  any  obvious  relation  to  the  conent  of  phosphoric  acid.1 

An  effort  has  been  made  to  ascertain  the  mineral  concentra- 
tion of  soil  solutions  as  they  occur  naturally  in  the  field.  Be- 

1  For  the  literature  of  the  earlier  work  on  the  composition  of  aqueous 
extracts  of  soils,  see :  How  crops  feed,  by  Samuel  W.  Johnson,  1890, 
p.  309  et  seq. ;  see  also,  On  the  analytical  determination  of  probably  avail- 
able "mineral"  plant-food  in  soils,  by  Bernard  Dyer,  Jour.  Chem.  Soc.,  65, 
115-167  (1894)  ;  and  Soils,  by  E.  W.  Hilgard,  1906,  p.  327  et  seq. 


THE   MINERAL   CONSTITUENTS   OF  THE  SOIL   SOLUTION         4! 

cause  of  the  practical  impossibility  of  extracting  the  actual  soil 
solution,  an  empirical  method  was  employed.  Areas  were  se- 
lected where  good  and  poor  crops  were  growing  near  each  other 
on  the  same  soil  types,  and  preferably  in  the  same  field.  Sam- 
ples of  soil  from  under  these  crops  were  taken  at  several  inter- 
vals during  the  growing  season,  quickly  removed  to  a  nearby 
laboratory,  shaken  thoroughly  with  distilled  water  in  the  pro- 
portion of  one  part  of  soil  to  five  parts  of  water,  allowed  to 
stand  twenty  minutes  and  the  supernatant  solution  passed  through 
a  Pasteur-Chamberland  filter.1 

As  has  been  pointed  out  above,  the  aqueous  extract  of  a  soil 
thus  arbitrarily  prepared  has  no  definite  or  casual  relation  to 
the  soil  solution  in  the  field.  It  is  certain  that  the  solutions 
would  not  generally  be  the  same.  It  should  also  be  emphasized 
that  such  a  procedure  can  not,  as  some  investigators  have  as- 
sumed, afford  a  criterion  between  soluble  and  insoluble  salts  in 
the  soil,  else  the  proportion  of  water  to  soil  used  above  some 
minimum  would  be  immaterial  as  far  as  the  amounts  which 
go  into  solution  are  concerned.  The  proportion  of  water  to 
soil  is  not  immaterial,  however,  considering  the  chemical  nature 
of  the  soil  components  and  the  results  of  experiment.  Con- 
sequently, it  is  clear  that  the  concentration  of  the  soil  solu- 
tion is  not  simply  the  ratio  of  the  amounts  found  in  the  aqueous 
extract,  to  the  percentage  of  moisture  in  the  soil,  but  something 
quite  different. 

Artificial  solutions  prepared  in  the  manner  described  above 
should,  however,  furnish  evidence  as  to  whether  or  not  there 
are  recognizable  differences  in  the  soluble  mineral  constituents 
of  good  and  poor  soils  respectively;  and  if  such  differences  exist, 
whether  they  are  consistent.  That  is  to  say,  if  the  more  pro- 
ductive soils  also  uniformly  yield  aqueous  extracts  of  a  higher 
concentration,  then  it  would  be  a  fair  inference  that  their  natural 
soil  solutions  are  maintained  at  a  higher  concentration  than  in 
the  less  productive  soils. 

1  Capillary  studies  and  filtration  of  clays  from  soil  solutions,  by 
Lyman  J.  Briggs  and  Macy  H.  Lapham,  Bull.  No.  19,  Bureau  of  Soils, 
U.  S.  Dept.  Agriculture,  1902 ;  Colorimetric,  turbidity  and  titration  methods 
used  in  soil  investigations,  by  Oswald  Schreiner  and  George  H.  Failyer, 
Bull.  No.  31,  Bureau  of  Soils,  U.  S.  Dept.  Agriculture,  1906. 
4 


THE   SOU,   SOLUTION 


Results  obtained  for  several  localities  and  several  crops,  taken 
from  the  original  records,  are  given  in  the  following  tables.1 

WATER  SOLUBLE  CONSTITUENTS  OF  SOIL. 

Locality,  Salem,  N.  J.     Soil  type,  Norfolk  sand.     Crop,  wheat. 
Yield,  good. 


Date 

Depth 
inches 

Moisture 
content 
Per  cent 

Parts  per  million  of  over-dried  sou 

Phosphoric 
acid  (PO4) 

Calcium 
(Ca) 

Potassium 
(K) 

0-12 
12-24 
1-24 
1-24 
1-24 

13-2 

"•5 

4-3 
4.6 
9.6 

12 
7 
4 
5 

2 

5 
5 

14 
13 
14 

12 

16 

13 
17 
24 

Tune  8  

Jnnp  Ti  .  . 

Locality,  Salem,  N.  J.     Soil  type,  Norfolk  sand.     Crop,  wheat. 
Yield,  ooor. 


Date 

Depth 
inches 

Nfoisture 
content 
Per  cent. 

Parts  per  million  of  oven-dried  soil 

Phosphoric 
acid  (PO4) 

Calcium 
(Ca) 

Potassium 
(K) 

Anril  1  .  . 

0-12 
12-24 
1-24 

12.0 
12.0 
9-3 

II 
10 

4 

5 
3 
29 

32 
22 
20 

Locality,  Salem,  N.  J.     Soil  type,  Sassafras  loam.     Crop,  wheat. 
Yield,  medium. 


Date 

Depth 
inches 

Moisture 
content 
Per  cent. 

Parts  per  million  of  over-dried  soil 

Phosphoric 
add(PO4) 

Calcium 
(Ca) 

Potassium 
(K) 

0-12 
12-24 
0-12 
12-24 
24-36 
0-12 
12-24 
24-36 
1-24 

23-2 
21.6 
22.3 
20.2 
20.3 
19-3 

18.6 

12.6 

22.5 

19 
II 
1  8 

15 

18 

7 
4 
5 
4 

to 

10 

8 

12 

«7 

10 

ii 

12 

14 

8 

»4 
IS 

21 

16 

21 

21 
21 
23 

1  The  chemistry  of  the  soil  as  related  to  crop  production,  by  Milton 
Whitney  and  F.  K.  Cameron,  Bull.  No.  22,  Bureau  of  Soils,  U.  S.  Dept. 
Agriculture,  1903. 


THE   MINERAL  CONSTITUENTS  OF  THE  SOU,  SOLUTION         43 


Locality,  Salem,  N.  J.     Soil  type,  Sassafras  loam.     Crop,  grass. 
Yield,  fair. 


Date 

Depth 
inches 

Moisture 
content 
Per  cent. 

Parts  per  million  of  oven-dried  soil 

Phosphoric 
acid(PO4) 

Calcium 
(Ca) 

Potassium 
(K) 

0-12 
12-24 
24-36 
0-12 
12-24 
24-36 
O-12 
12-24 
0-12 
12-24 
24-36 

25.0 
23-8 
19.9 
25-8 
23.1 
21.8 

23.0 

21.6 

24.8 

24.0 

21.4 

13 
7 
16 
21 

8 

9 
ii 

8 
8 
6 
3 

28 
26 
8 

12 
12 
15 
23 
20 

16 

21 
II 

18 
13 
15 
21 

15 
21 

43 
34 
4i 
38 
25 

Locality,  Salem,  N.  J.     Soil  type,  Sassafras  loam.     Crop,  wheat. 
Yield,  good. 


Date 

Depth 
inches 

Moisture 
content 
Per  cent. 

Phosphoric 
add(PO4) 

Calcium 
(Ca) 

Potassium 
(K) 

0-12 
12-24 
O-I2 
12-24 
0-12 
12-24 
O-I2 
12-24 
24-36 
0-12 
12-24 
24-36 
0-12 
12-24 
1-24 
1-24 
1-24 
1-24 
1-24 

22.0 

18.1 
18.3 
18.  i 
24.7 
22.3 
23-4 
23-9 
22.4 
25-6 
24.4 

21.6 
5-2 

8.0 
10.6 
15-5 

8.2 

15-0 

10.6 

8 
8 

10 

9 
14 
8 

4 

12 

8 
8 
8 
8 
H 
15 

2 

6 
6 
5 
7 

6 
15 
15 
24 
12 

li     - 

16 

16 

3 
16 

17 
ii 

5i 
55 

20 

26 

19 
21 

63 

10 

14 

Lost 
25 
30 
38 
16 

20 
21 
30 

47 

38 
23 
32 
13 
14 

22 
19 

17 

March  26  ....... 

44 


THE  SOIL   SOLUTION 


Locality,  Salem,  N.  J.     Soil  type,  Sassafras  loam.     Crop,  clover. 
Yield,  fair. 


Date 

Depth 
inches 

Moisture 
content 
Per  cent. 

Parts  per  million  of  oven-dried  soil 

Phosphoric 
acid  (PO4) 

Calcium 
(Ca) 

Potassium 
(K) 

0-12 

12-24 
24-36 
0-12 
12-24 
24-36 
0-12 
12-24 

20.8 
20.2 

18.6 

26.8 

22.9 

22.5 

8.1 
12.7 

5 
5 
5 
9 
8 

4 
8 

9 

15 
15 
12 

31 
20 

H 

16 

18 

32 
27 
36 
2O 

18 

20 

17 
2O 

March  26  

Locality,  St.  Marys,  Md. 


Soil  type,  Leonardtown  loam.     Crop,  wheat. 
Yield,  good. 


Date 

Depth 
inches 

Moisture 
content 
Per  cent. 

Parts  per  million  of  oven-dried  soil 

Phosphoric 
acid(PO4) 

Calcium 
(Ca) 

Potassium 
(K) 

0-12 

12  24 
0-12 
12-24 
O-I2 
12-24 
O-I2 
12-24 
0-12 
12-24 
O-I2 
12-24 
0-24 
0-24 
0-24 

21.8 
21.3 
22.2 
21.8 
22.4 
21.8 

17.0 

21.0 
15-0 

15-9 
14-2 
I9.9 
15-0 
15-7 
16.4 

5 
4 
8 

4 
7 
7 
5 
5 
13 
9 
3 
4 
6 

IO 

7 
*5 
ii 

H 
8 
16 
7 
34 
17 
H 
13 
ii 

3 
15 

12 
10 
52 
38 
23 
30 
25 
19 
28 
26 

24 

25 
13 
17 
15 

May  I  

MAV  o   .  . 

may  y 

THE   MINERAL   CONSTITUENTS  OF   THE   SOU,   SOLUTION         45 


Locality,  St.  Marys,  Md.     Soil  type,  Leonardtown  loam.     Crop,  wheat. 

Yield,  poor. 


Date 

Depth 
inches 

Moisture 
content 
Per  cent. 

Parts  per  million  of  oven-dried  soil 

Phosphoric 
acid(PO4) 

Calcium 
(Ca) 

Potassium 

(K) 

May  14  

0-12 
12-24 
0-12 

12-24 
0-24 
0-24 

14.7 
19.9 
7.8 
14.9 
16.0 
19-5 

5 

4 
4 
4 
4 
6 

8 
4 
7 
II 

4 

4 

35 
30 

22 
23 

16 

13 

August  14  

Locality,  St.  Marys,  Md.     Soil  type,  Leonardtown  loam.     Crop,  corn. 

Yield,  good. 


Date 

Depth 
inches 

Moisture 
content 
Per  cent. 

Parts  per  million  of  oven-dried  soil 

Phosphoric 
acid  (  PO4) 

Calcium 
(Ca) 

Potassium 
(K) 

May  8  

O-I2 
12-24 
0-12 
12-24 
0.24 

18.2 

18.9 

18.2 
18.8 
17-5 

9 

10 

3  - 
6 

7 

12 

7 

24 
19 
30 

29 
26 

38 
28 
18 

May  18  

Locality,  St.  Marys,  Md.     Soil  type,  Leonardtown  loam.     Crop,  corn. 

Yield,  poor. 


Date 

Depth 
inches 

Moisture 
content 
Per  cent. 

Parts  per  million  of  oven-dried  soil 

Phosphoric 
acid  t,PO4) 

Calcium 
(Ca) 

Potassium 
(K.) 

0-12 

12-24 
0-24 
0-24 

16.6 

17.4 
19.9 

21.6 

5 
6 

9 

7 

12 

8 

25 
15 

22 
22 
20 
13 

Aupimt  T^.  . 

It  will  be  observed  that  the  results  given  in  the  above  tables 
are  expressed  in  parts  per  million  of  oven-dried  soils,  in  order 
to  have  some  definite  basis  of  comparison,  and  because  it  was 
anticipated  at  the  time  the  investigation  was  made  that  larger 


46  THE  son,  SOLUTION 

quantities  of  dissolved  minerals  would  be  found  under  the  better 
crops,  and  vice  versa.  An  inspection  of  the  results,  however, 
shows  that  no  such  correlation  can  be  made,  nor  in  fact  can  any 
consistent  correlation  be  made  between  the  dissolved  material 
and  crop,  soil  type,  water  content,  depth  of  soil  or  part  of  the 
growing  season.1  It  appears,  therefore,  that  in  so  far  as  the 
field  method  of  analyzing  an  arbitrarily  prepared  aqueous  ex- 
tract is  competent,  there  is  no  evidence  that  there  are  important 
characteristic  differences  in  the  concentration  of  the  mineral 
constituents  in  different  soil  solutions  in  the  field. 

The  order  of  concentration  of  the  soil  solution  can  be  approxi- 
mated from  the  given  data,  if  the  assumption  be  made  that  in 
the  preparation  of  the  aqueous  extract,  soluble  mineral  constit- 
uents are  of  minor  importance,  other  than  the  constituents 
already  dissolved  in  the  soil  solution.  The  calculation  is  very 
laborious,  is  not  exact,  and  on  account  of  the  assumptions  made 
the  actual  figures  obtained  are  of  no  especial  value  in  any  par- 
ticular case.  Remembering  the  method  of  making  up  the  solu- 
tions from  which  these  results  were  obtained,  it  would  be  suffi- 
ciently near  the  truth  to  assume  an  average  moisture  content 
of  20  per  cent.,  when  the  figures  given  here  for  the  soil  approxi- 
mate those  which  would  be  obtained  for  the  soil  solution.  More 
exact  calculations  have  been  made  for  a  large  number  of  such 
cases,  and  it  has  been  found  from  this  method  of  estimation 
that  the  average  composition  with  respect  to  phosphoric  acid 
would  be  about  6-8  parts  per  million,  and  for  potash  about 
25  parts  per  million,  figures  which  agree  with  the  results  ob- 
tained for  the  examination  of  solutions  extracted  from  saturated 
soils  by  means  of  the  centrifuge. 

1  King,  however,  claims  that  the  concentration  of  the  soil  solution 
with  respect  to  mineral  plant  nutrients,  is  higher  in  the  soils  of  the 
northern  states  than  in  the  soils  of  the  South  Atlantic  states.  See : 
Some  results  of  investigations  in  soil  management,  by  F.  H.  King,  Year- 
book, U.  S.  Dept.  Agriculture,  1903,  pp.  159-174.  Bailey  E.  Brown  has 
obtained  some  preliminary  results  which  suggest  that  there  may  be 
seasonal  variations  with  respect  to  some  of  the  dissolved  mineral  con- 
stituents. See,  Annual  Report  of  the  Pennsylvania  State  Experiment 
Station,  1908-9,  pp.  31  et  seq. 


THE   MINERAL   CONSTITUENTS   OF  THE   SOU,   SOLUTION         47 

The  results  given  in  the  foregoing  tables  were  obtained  under 
great  difficulties,  and  in  some  parts  the  variations  they  show  are 
undoubtedly  due  to  inevitable  inaccuracies  of  analytical  work 
done  under  such  circumstances.  Some  of  the  variations  may 
also  be  due  to  the  disturbing  influences  in  the  soil  referred  to 
above.  Experience  has  shown,  however,  that  the  preparation 
of  an  aqueous  extract  of  the  soil  of  any  particular  field  is  by  no 
means  a  simple  matter.  Extracts  made  from  samples  taken 
within  a  few  feet  of  one  another  frequently  show  variations  of 
the  same  order  as  with  samples  from  entirely  different  fields,  or 
even  soil  types.  Differences  in  the  preliminary  drying  out  of 
the  sample  before  the  addition  of  the  water,  seems  to  result  in 
the  same  order  of  differences  as  obtained  between  different  soils. 
In  consequence  of  these  facts,  and  of  the  further  fact  that  an 
arbitrary  aqueous  extract  of  a  soil  cannot  be  assumed  to  repre- 
sent in  any  definite  way  the  natural  soil  solution,  the  results  of 
the  field  examination  are  inconclusive  as  to  the  concentration  of 
the  soil  solution  in  situ.  It  is  more  necessary,  therefore,  that 
other  lines  of  evidence  should  be  sought  as  to  the  mineral  char- 
acteristics and  concentration  of  the  soil  solution.  Such  a  line 
of  evidence  is  found  in  certain  percolation  experiments.1 

If  a  solution  of  a  soluble  phosphate  be  percolated  through  a 
soil,  a  part  of  the  phosphate  will  be  removed  from  the  solution 
and  absorbed  by  the  soil;  that  is,  there  will  be  a  redistribution 
of  the  phosphate  between  the  soil  and  the  water.  As  the  pro- 
cess continues,  however,  relatively  less  and  less  phosphate  is 
absorbed  by  the  soil  and  the  concentration  of  the  percolate  be- 
comes more  and  more  nearly  that  of  the  added  solution.  This 
absorption  takes  place  more  or  less  closely  in  accordance  with 
the  simple  law  that  the  absorption  of  phosphates  by  the  soil, 
per  unit  of  solution  which  is  percolating,  is  proportional  to  the 
total  amount  of  phosphate  which  the  soil  may  yet  take  from 
that  solution  if  percolated  indefinitely.  This  law  is  expressed 
by  the  equation  dy/dx  =  K(A  —  y)  where  3;  is  the  amount 

1  The  absorption  of  phosphates  and  potassium  by  soils,  by  Oswald 
Schreiner  and  George  H.  Failyer,  Bull.  No.  32,  Bureau  of  Soils,  U.  S. 
Dept.  Agriculture,  1906. 


48  THE   SOIL   SOLUTION 

absorbed,  x  amount  of  solution  that  has  passed,  and  A  is  the 
total  amount  which  can  ultimately  be  absorbed  by  that  particular 
soil  from  that  particular  solution.  K  is  also  a  characteristic 
constant.  If  the  percolation  be  maintained  at  constant  rate,  then 
/,  time,  can  be  substituted  for  x  and  the  equation  becomes 
dy/dt  =  K(A  —  y),  the  ordinary  rate  equation  for  a  mono- 
molecular  reaction  of  the  first  order,  whether  chemical  or  phys- 
ical. 

With  such  absorptions  as  are  involved  in  soils,  a  clay  exposes 
a  greater  amount  of  absorbing  surface  than  does  a  loam  or  sand, 
and  it  will  show  the  greatest  absorption  towards  any  particular 
solution,  other  things  being  equal.  The  curve  showing  the  con- 
centration of  percolate  would  lie  lower  for  a  clay  than  for  a 
loam,  or  for  a  sand.  This  is  illustrated  in  the  accompanying 
sketch  diagram,  where  y  represents  concentration  of  percolate 
and  t  represents  time. 


Fig.  i. 

If  after  percolation  has  proceeded  for  some  time  (in  some 
experiments  for  several  weeks  and  until  the  soil  contained  I 
or  2  per  cent,  of  phosphoric  acid)  pure  water  be  passed  through 
the  soil,  then,  as  soon  as  the  previously  used  phosphate  solution 
has  been  displaced,  the  concentration  of  the  percolate  drops  and 
continues  practically  constant  for  an  indefinite  period.  More- 
over, no  matter  what  the  soil  may  be  as  to  texture  or  compo- 
sition, the  same  concentration  of  percolate  is  obtained,  namely, 
6-8  parts  per  million,  the  concentration  which  the  soils  yielded 
prior  to  treatment  with  the  phosphate  solution.  Similar  ex- 
periments when  the  soils  were  treated  with  salts  of  potassium 
have  given  like  results,  although  the  curves  obtained  from  pass- 
ing pure  water  through  the  soils  do  not  lie  quite  so  close  to- 


THE   MINERAL   CONSTITUENTS   OF   THE  SOIL   SOLUTION         49 

getter;  but  the  concentration  of  the  percolate  with  respect  to 
potassium  generally  lies  somewhere  between  25  and  30  parts  per 
million. 

The  removal  of  a  soluble  constituent  from  the  soil  by  per- 
colating water  appears  to  be  described  by  a  rate  equation  similar 
to  that  given  above  for  absorption.  If  the  rate  of  percolation 
be  maintained  constant  this  formula  is 

dx/dt  =  K(B  —  x) 

where  x  is  the  amount  removed  by  the  percolation,  with  time 
t,  K  is  a  constant  characteristic  for  the  particular  system  under 
consideration,  and  B  is  the  total  amount  of  the  constituent 
which  may  ultimately  be  leached  out.  In  other  words,  the  rate 
in  any  particular  soil  will  depend  upon  the  amount  of  the  con- 
stituent still  absorbed  in  that  soil  but  has  no  necessary  connec- 
tion with  the  rate  which  would  hold  for  the  same  amount  of 
the  constituent  in  any  other  soil. 

Theoretically,  two  consequences  follow  from  this  law  which 
require  consideration  here.  The  rate  at  which  a  constituent  is 
removed  gradually  becomes  less  as  percolation  proceeds.  If  the 
soil  contains  an  amount  of  the  constituent  approaching  the  total 
amount  which  it  can  absorb,  as  for  instance  is  probably  the  case 
sometimes  when  large  applications  of  lime  have  been  made  to  the 
soil,  the  concentration  of  the  percolating  solution  might  be  ex- 
pected to  change  noticeably.  Generally,  however,  a  soil  con- 
tains nowhere  near  as  much  phosphoric  acid  or  potassium  as  it 
is  capable  of  absorbing,  so  that  the  concentration  of  the  percolat- 
ing water  changes  but  very  little  with  respect  to  these  constit- 
uents. It  follows  from  the  equation  that  if  percolation  con- 
tinues uninterrupted,  the  concentration  of  the  percolate,  so  far 
as  it  is  determined  by  an  absorbed  constituent,  must  get  less 
and  less  until  it  becomes  a  vanishing  quantity.  This  state  of 
affairs  does  not  exist  in  the  soil,  however,  for  percolation  by 
pure  water  does  not  continue  uninterrupted  for  any  length  of 
time.  The  rise  of  the  capillary  water  in  the  soil  will,  under 
normal  conditions,  enable  the  soil  to  reabsorb  more  of  the  or- 
dinary mineral  constituents  than  is  removed  by  percolating 


50  THE  SOIL  SOLUTION 

waters.     Further  attention  will  be  given  the  matter  in  another 
chapter. 

Another  but  quite  different  line  of  evidence  as  to  the  probable 
concentration  of  the  soil  solution  is  furnished  by  the  investiga- 
tion of  the  solubility  of  certain  phosphates.1  It  is  popularly 
supposed  that  when  superphosphate  containing  mono-calcium 
phosphate,  CaH4(PO4)2.H2O,  is  added  to  a  soil  there  is  a  more 
or  less  permanent  increase  of  readily  soluble  phosphoric  acid  in 
the  soil,  although  a  part  "inverts"  to  the  somewhat  less  soluble 
dicalcium  phosphate,  CaH(PO4).2H2O.  Such  probably  is  far 
from  a  correct  view  of  what  actually  takes  place.  The  results 
obtained  by  studying  the  solubility  of  the  different  lime  phos- 
phates in  water  at  ordinary  temperature  (25°  C.)  can  be  ex- 
pressed in  a  diagram  similar  to  the  accompanying  sketch,  which 
is  much  distorted  for  convenience  in  lettering.  As  the  diagram 
indicates,  when  the  concentration  of  the  solution  increases  with 
respect  to  phosphoric  acid,  the  lime  is  at  first  less  and  less  solu- 
ble and  the  point  represented  by  B  is  reached,  then  becomes 
more  and  more  soluble  until  the  point  D  is  reached,  from  then 
on  becoming  less  and  less  soluble,  until  the  solution  reaches  a 


P205  in  solution 


Fig.  2. 

syrupy  consistency.     In  contact  with  all  solutions  represented 
by  points  on  the  line  DH  the  stable  solid  substance  which  can 

1  For  reference  to  the  literature  and  detailed  discussion  see :  The 
action  of  water  and  aqueous  solutions  upon  soil  phosphates,  by  F.  K. 
Cameron  and  J.  M.  Bell,  Bull.  No.  41,  Bureau  of  Soils,  U.  S.  Dept.  Agri- 
culture, 1907. 


THE   MINERAL   CONSTITUENTS   OF  THE  SOIL   SOLUTION         51 

exist  in  mono-calcium  phosphate,  CaH4(PO4)2.H2O.  Along  the 
line  CD  the  only  solid  which  is  stable  and  can  continue  to  per- 
sist is  the  dicalcium  phosphate.  From  the  point  C  the  composi- 
tion of  the  stable  solid  varies  continuously  with  the  concentra- 
tion of  the  liquid  solution.  Therefore,  these  solids  form  a 
series  varying  in  composition  from  pure  dicalcium  phosphate 
to  pure  calcium  hydroxide.  One  of  these  basic  phosphates,  as 
they  would  ordinarily  be  called,  has  a  less  solubility  than  any 
other,  as  indicated  by  the  point  B.  All  solutions  to  the  right  of 
the  point  B  have  an  acid  reaction,  while  all  solutions  to  the  left 
possess  an  alkaline  reaction.  It  follows  from  these  facts  that 
if  we  start  with  any  lime  phosphate  corresponding  to  some 
point  to  the  right  of  B  and  dilute  it,  or  what  amounts  to 
the  same  thing  in  case  it  has  been  added  to  the  soil,  if  we 
leach  it,  phosphoric  acid  will  go  into  solution  more  rapidly 
than  will  lime  until  the  composition  of  the  residue  is  that  of 
the  basic  phosphate  stable  at  B:  Similarly,  if  we  start  with  a 
phosphate  more  basic,  lime  will  be  removed  more  rapidly  than 
phosphoric  acid,  until  the  residue  has  the  composition  of  the 
phosphate  of  lowest  solubility.  From  this  point,  with  continued 
leaching,  the  lime  and  phosphoric  acid  will  dissolve  in  a  definite 
ratio,  which  ratio  is  obviously  that  of  the  phosphate  of  least  solu- 
bility. That  is  to  say,  if  the  leaching  process  is  slow,  as  would 
be  the  case  under  soil  conditions,  the  solution  would  have  a 
perfectly  definite  concentration  with  respect  to  lime  and  phos- 
phoric acid.  What  the  ratio  of  lime  to  phosphoric  acid  may 
be,  is  of  no  particular  interest  in  this  connection,  but  the  order 
of  concentration  of  phosphoric  acid  is  of  interest.  Owing  to 
serious  analytical  difficulties,  this  has  not  yet  been  determined 
with  any  great  precision,  but  by  interpolating  on  the  experiment- 
ally determined  curve  AC,  this  concentration  is  found  to  be  some- 
where in  the  neighborhood  of  5-10  parts  per  million,  figures 
close  to  those  obtained  for  the  concentration  of  the  soil  solution 
with  respect  to  phosphoric  acid  by  the  previously  described -in- 
vestigations. 

Under  ordinary  circumstances,  however,  it  is  not  probable  that 
lime  is  the  dominant  base  controlling  the  concentration  of  phos- 


52  THE   SOIL   SOLUTION 

phoric  acid  in  the  soil  solution,  since  the  great  majority  of  ag- 
ricultural soils  contain  vastly  more  ferric  oxide  (more  or  less 
hydrated)  than  is  equivalent  to  any  amount  of  phosphoric  acid 
that  will  ever  be  brought  into  the  soil ;  and  ferric  phosphates  are 
less  soluble  relatively  than  lime  phosphates.  Investigation  of 
the  relation  of  ferric  oxide  to  solutions  of  phosphoric  acid  shows 
that  the  system  is  quite  similar  in  many  respects  to  the  basic 
lime  phosphates  and  water  just  described.  When  the  ratio  of 
iron  to  phosphoric  acid  in  the  solid  is  greater  than  that  required 
by  the  formula  of  the  normal  phosphate,  FePO4,  the  aqueous 
solution  will  have  an  acid  reaction  and  contain  a  mere  trace  of 
iron  and  an  amount  of  phosphoric  acid  determined  by  the  com- 
position of  the  solid  and  by  the  proportion  of  solid  to  water. 
The  basic  ferric  phosphates  seem  to  be  solid  solutions  which 
yield  a  very  dilute  aqueous  solution  when  brought  into  contact 
with  water.  What  the  concentration  will  be  under  soil  condi- 
tions is  shown  by  the  percolation  experiments  cited  above. 

The  addition  of  other  substances  will  in  many  cases  affect 
more  or  less  the  solubility  of  the  soil  minerals.  If  these  sub- 
stances be  electrolytes,  they  will  generally,  but  not  always,  affect 
the  solubility  of  the  minerals  as  would  be  anticipated  from  the 
hypothesis  of  electrolytic  dissociation.  Thus,  the  addition  of 
potassium  sulphate  lessens  the  solubility  and  hydrolysis  of  a 
potash  feldspar  or  a  potash  mica.  Contrary,  however,  to  the 
indications  of  the  hypothesis,  sodium  nitrate  decreases  the  solu- 
bility of  a  ferric  phosphate.  While  appreciable  solubility  effects 
take  place  with  sufficiently  high  concentrations,  laboratory  ex- 
periments indicate  that  the  addition  of  such  substances,  even  in 
a  liberal  application  of  fertilizers,  is  not  sufficient  to  produce 
any  great  effect  on  the  concentration  of  the  soil  solution.  Simi- 
larly, it  has  often  been  supposed  that  the  ammonia,  and  nitrous 
and  nitric  oxides  of  the  atmosphere  carried  into  the  soil  by  rain, 
or  formed  in  the  soil  by  bacterial  action,  affect  the  solubility  of 
the  soil  minerals,  but  it  is  highly  improbable  that  the  concentration 
with  respect  to  these  agents  ever  becomes  sufficiently  high,  as 
laboratory  investigations  show  to  be  necessary  to  affect  appreci- 
ably the  solubility  of  the  ordinary  rock-  or  soil-forming  minerals. 


53 

Rain  brings  from  the  atmosphere  into  the  soil  two  agents, 
however,  which  do  markedly  affect  the  solubility  of  the  soil  min- 
erals, namely,  oxygen  and  carbon  dioxide.  The  atmosphere 
within  the  soil  contains  normally  a  somewhat  smaller  proportion 
of  oxygen  than  does  the  air  above  the  soil.  Rain  in  falling 
through  the  air  absorbs  or  dissolves  relatively  more  oxygen  than 
nitrogen.  Therefore  when  the  rain  water  has  penetrated  the 
soil  to  any  considerable  depth  there  should  be,  and  probably  is, 
a  liberation  of  dissolved  oxygen  into  the  atmosphere  of  the  soil 
interstices.  This  dissolved  oxygen  in  becoming  liberated  or  when 
dissolved  in  the  film  water  appears  to  be  especially  active  to- 
wards the  ferrous  or  ferro-magnesian  silicates.  These  minerals 
are,  moreover,  as  a  class  probably  the  most  soluble  of  the  rock- 
forming  silicates.  Consequently  oxygen  brought  into  the  soil 
in  this  manner  is  one  of  the  most  important  agencies  in  breaking 
down  and  decomposing  such  minerals  as  the  amphiboles,  pyrox- 
enes, chlorites,  certain  serpentines,  phlogopites  and  biotites ;  at  the 
same  time  there  is  formed  ferric  oxide  (more  or  less  hydrated) 
and  silica  (probably  as  quartz)  and  magnesium,  potassium,  cal- 
cium or  sodium  pass  into  solution,  probably  as  bicarbonates. 
That  the  concentration  of  the  soil  moisture  may  thus  be  made 
temporarily  abnormal  is  not  impossible,  though  scarcely  prob- 
able. 

The  soil  atmosphere  has  normally  a  decidedly  higher  content 
of  carbon  dioxide  than  the  atmosphere  above  the  soil.  Conse- 
quently the  soil  water  is  always  more  or  less  "charged"  with 
carbon  dioxide,  and  the  presence  of  the  carbon  dioxide  decidedly 
augments  the  solvent  powers  of  the  water  towards  a  great  many 
and  different  kinds  of  rock-forming  or  soil  minerals.1 

What  the  mechanism  of  the  reaction  may  be  is  far  from  clear. 

1  For  references  to  the  literature  see  Bull.  No.  30,  Bureau  of  Soils, 
U.  S.  Dept.  of  Agriculture;  also,  The  action  of  carbon  dioxide  under 
pressure  upon  a  few  metal  hydroxides  at  o°  C.,  by  F.  K.  Cameron  and 
W.  O.  Robinson,  Jour.  phys.  chem.,  12,  561-573  (1908);  The  influence 
of  colloids  and  fine  suspensions  on  the  solubility  of  gases  in  water, 
Part  I.  Solubility  of  carbon  dioxide  and  nitrous  oxide,  by  Alexander 
Findlay  and  Henry  Jermain  Maude  Creighton,  Trans.  Chem.  Soc.,  97, 
536-561  (1910). 


54  THE  SOII,  SOLUTION 

The  obvious  explanation,  at  least  in  the  case  of  the  ordinary 
silicates  of  the  alkalis  or  alkaline  earths,  is  that  by  forming  bi- 
carbonates  of  the  hydrolyzed  bases,  the  active  mass  of  the  re- 
action product  with  water  is  decreased  and  hydrolysis  thereby 
increased.  But  this  explanation  is  apparently  insufficient  to  ac- 
count for  the  effects  sometimes  observed.  It  has  been  shown 
that  the  passage  of  carbon  dioxide  through  solutions  of  the 
silicates,  will  produce  more  or  less  slowly  a  precipitation  of  silica, 
and  there  seems  little  reason  to  doubt  that  it  does  induce  to 
some  degree  a  decomposition  and  consequent  greater  solubility 
of  the  silicates  of  the  alkalis  and  alkaline  earths.  It  also  in- 
creases to  an  appreciable  extent  the  solubility  of  the  phosphates 
of  iron,  alumina,  and  lime.  Therefore,  the  variation  in  the  con- 
tent of  carbon  dioxide  in  different  soils,  and  its  continual  varia- 
tion from  time  to  time  in  any  one  soil,  must  be  expected  to  pro- 
duce corresponding  changes  in  the  soil  solution  with  respect  to 
such  bases  as  potassium  and  lime,  and  also  with  respect  to  phos- 
phoric acid.  This  has  been  verified  experimentally  with  aqueous 
extracts  of  soils,  the  solutions  being  charged  with  carbon  dioxide 
while  in  contact  with  the  soils.1  It  is  not  conceivable,  however, 
that  any  great  difference  can  exist  in  the  partial  pressures  of 
carbon  dioxide  in  different  soils  which  are  in  a  condition  to 
support  crops,  and  therefore  great  absolute  differences  in  the 
mineral  content  of  the  soil  solution  are  not  to  be  anticipated, 
nor  are  they  actually  observed. 

It  has  long  been  held  that  the  organic  substances  in  the  soil 
have  an  important  solvent  effect  on  the  minerals.  This  assump- 
tion seems  quite  unwarranted  in  the  light  of  our  present  knowl- 
edge, although  it  is  not  to  be  denied  that  occasionally  there 
may  be  present  in  the  soil  some  soluble  organic  substance  which 
influences  the  mineral  content.  Generally  it  has  been  assumed 
that  the  effective  organic  substances  influencing  the  solubility 
of  the  minerals  are  organic  acids,  of  which  a  number  have  found 
their  way  into  past  and  even  current  literature,  and  which  have 
1  See,  for  instance,  the  results  obtained  by  Peter,  Proceedings  of  the 
I9th  Annual  Convention  of  the  Association  of  American  Agricultural 
Colleges  and  Experiment  Stations,  Bull.  No.  164,  Office  of  Experiment 
Stations,  U.  S.  Dept.  Agriculture,  1906,  p.  151  et  seq. 


THE;  MINERAL  CONSTITUENTS  OF  THE  son,  SOLUTION       55 

been  designated  as  humic,  ulmic,  crenic,  apocrenic,  azohumic 
acids,  etc.  Their  existence  has  been  predicated  upon  two  facts: 
First,  humus  is  soluble  in  alkaline  solutions  but  is  more  or  less 
completely  reprecipitated  on  the  addition  of  an  excess  of  a  strong 
mineral  acid,  a  phenomenon  also  characteristic  of  many  organic 
acids.  But  many  other  organic  substances  than  acids  are  also 
soluble  in  the  presence  of  alkalis  and  insoluble  in  the  presence 
of  an  excess  of  strong  mineral  acids.  .Second,  organic-copper 
complexes  have  been  obtained  from  humus  constituents,  and 
supposed  to  be  copper  salts  of  various  humus  acids.  The  de- 
scriptions of  these  complexes  so  far  given  do  not  show  that  they 
met  the  usual  criteria  for  definite  compounds,  but  indicate  on 
the  contrary  that  they  were  the  results  of  absorption  or  possibly 
adsorption  phenomena.  Consequently  the  existence  of  "humic" 
acids  is  purely  hypothetical  and  without  experimental  or  other 
scientific  verification,  and  calls  for  no  further  consideration  here. 
It  is  a  widespread  and  popular  notion  that  substances  with 
a  slight  solubility  also  dissolve  slowly,  and  that  consequently 
the  solubility  of  the  minerals  in  the  soil  water  must  necessarily 
be  a  very  slow  process.  This  is,  however,  a  misapprehension. 
It  has  been  shown  with  a  number  of  the  common  rock-forming 
minerals,  that  if  they  be  powdered  and  then  stirred  into  a  rela- 
tively small  volume  of  water,  they  dissolve  very  rapidly  at  first, 
and  in  a  very  short  time,  generally  a  few  minutes,  the  solution 
is  nearly  saturated  with  respect  to  the  mineral.  Complete  sat- 
uration, however,  may  require  many  days.  The  general  shape 
of  curve  expressing  the  rate  of  solubility  is  shown  in  the  ac- 
companying figure.1  For  soils,  this  fact  has  been  verified  re- 
peatedly, in  the  following  way :  A  cell  fitted  with  parallel  elec- 
trodes is  placed  in  circuit  with  a  slide-wire2  or  Wheatstone 
bridge  in  such  a  manner  that  the  resistance  of  the  cell  contents 
can  be  quickly  determined.  Distilled  water  is  then  placed  in 
the  cell  and  its  resistance  found.  Generally  this  will  be  up- 

1  See,    for    example,    Umwandlung    des    Feldspats    in    Sericit    (Kali- 
glimmer)  von  Carl  Benedick,  Bull.  Geol.  Inst.  Upsala,  7,  278-286  (1904). 

2  See   Electrical  instruments   for  determining  the  moisture,  tempera- 
ture and  soluble  salt  content  of  soils,  by  L.  J.  Briggs,  Bull.  No.  15,  and 
The  electric  bridge   for  the  determination   of   soluble   salts   in   soils,   by 
R.  O.  E.  Davis  and  H.  Bryan,  Bull.  Noo.  61,   Bureau  of  Soils,  U.   S. 
Dept.  Agriculture. 


56  THE   SOIL   SOLUTION 

wards  of  1,000,000  ohms.  The  soil  or  rock  powder  under  ex- 
amination is  then  added  to  the  cell,  being  rapidly  stirred  into 
the  water  contained  therein.  The  resistance  drops  to  about 
5,000  ohms  with  a  short  space  of  time,  usually  three  or  four 
minutes.  A  further  slight  drop  in  the  resistance  generally  takes 
place,  but  it  requires  days,  and  sometimes  even  months  to  be- 
come more  than  barely  appreciable.  In  this  manner  it  has  been 


Time 


Fig-  3- 

shown  that  the  soil  and  many  of  the  common  soil  minerals 
dissolve  quite  rapidly  if  they  are  sufficiently  fine  to  offer  a  large 
surface  to  the  action  of  the  water.  It  would  seem  to  follow, 
therefore,  that  in  the  case  of  the  soil  solution  the  concentra- 
tion with  respect  to  these  constituents  derived  from  the  soil 
minerals,  will  be  rapidly  restored  whenever  disturbed  through 
absorption  by  plants,  leaching,  or  otherwise. 

That  the  minerals  of  the  soil,  or  a  powdered  mineral  or  rock- 
powder,  will  dissolve  continually  as  the  concentration  of  the  so- 
lution in  contact  with  it  is  disturbed  by  abstraction  of  a  dis- 
solved mineral  substance,  has  been  shown  by  numerous  experi- 
menters. An  apparently  obvious  way  to  test  this  point  would  be 
to  treat  che  soil  sample  with  successive  portions  of  water,  and 
to  analyze  the  successive  portions  for  the  dissolved  mineral  sub- 
stances. This  method,  however,  involves  serious  experimental 
difficulties,  owing  to  the  smaller  sized  mineral  particles  being 
suspended  in  the  mother  liquor,  thus  precluding  satisfactory  de- 
cantation  and  clogging  filters.  Moreover,  such  a  process  in  no 
case  simulates  field  conditions.  To  meet  these  difficulties,  the 
soil  or  mineral  powder  has  been  placed  between  two  porous 


THE   MINERAL   CONSTITUENTS  OF  THE   SOU,   SOLUTION         57 

media,  as  in  the  space  between  two  concentric  cylinders  of  un- 
glazed  porcelain,  the  space  being  closed  by  a  rubber  stopper. 
To  the  interior  cylinder  is  fitted  a  stopper  carrying  a  tube  of 
insoluble  metal,  such  as  platinum  or  tin.  This  tube  is  bent  into 
a  goose-neck  form,  and  just  below  the  stopper  the  tube  is  per- 
forated with  a  small  opening.  The  whole  apparatus  is  filled 
with  water  and  set  in  a  beaker,  also  filled  with  water.  The  metal 
tube  is  made  the  cathode  in  an  electric  circuit,  a  platinum  or 
other  suitable  anode  being  introduced  into  the  beaker.  In  a  few 
minutes  the  dissolved  and  hydrolyzed  bases  pass  into  the  cathode 
chamber,  and  as  the  water  also  accumulates  in  the  chamber  by 
electrolytic  endosmosis,  a  solution  of  the  bases  dissolved  from 
the  soil  minerals  drops  from  the  end  of  the  metal  goose-neck. 
By  adding  water  to  the  outer  beaker  from  time  to  time,  a  steady 
stream  of  alkaline  solution  has  been  obtained  for  months,  and 
in  no  case  yet  has  a  soil  thus  treated  failed  to  continue  to  yield 
up  the  bases  it  contains  in  its  mineral  particles.  The  acids,  such 
as  phosphoric  acid  for  example,  are  of  course  found  in  the 
water  outside  the  porous  cells,  and  in  the  case  of  the  phosphoric 
acid  it  also  appears  to  continue  indefinitely  to  be  withdrawn  from 
the  soil.1  It  thus  appears  that  as  the  products  of  solution  and 
hydrolysis  are  removed,  by  such  an  endosmotic  device  as  that 
just  described  or  by  the  roots  of  growing  plants,  by  leaching  or 
otherwise,  the  soil  minerals  will  continue  to  dissolve. 

The  foregoing  arguments  as  to  the  concentration  of  the  soil 
solution  with  respect  to  those  constituents  derived  from  the  soil 
minerals,  are  based  on  the  generally  recognized  principle  that  a 
material  system  left  to  itself  tends  towards  a  condition  of  stable 
equilibrium  or  final  rest,  that  is,  a  condition  where  such  changes 
as  are  taking  place  are  so  balanced  that  no  change  occurs  in 
the  system  as  a  whole.  But  the  soil  is  a  system  continually 
subject  to  outside  forces  and  influences,  and  as  pointed  out 
above,  is  of  necessity  a  dynamic  system.  It  is  doubtful  in  the 
extreme  if  any  soil  in  place  is  ever  in  a  state  of  final  stable 
equilibrium.  It  would  be  natural,  therefore,  to  expect  and  to 

1  For  detailed  description  of  the  apparatus  and  experimental  data,  see 
Bull.  No.-  30,  p.  27,  et  seq.,  Bureau  of  Soils,  U.  S.  Dept.  Agriculture. 

5 


58  THE  SOIL  SOLUTION 

find  that  even  if  the  solution  in  the  soil  were  dependent  on  the 
solubility  of  the  soil  minerals  alone  and  were  continually  tend- 
ing towards  a  definite  normal  concentration,  actually  this  con- 
centration would  seldom  if  ever  be  realized.  Most  important  in 
this  connection  is  the  fact  that  the  concentration  of  the  soil  so- 
lution is  always  dependent  in  some  degree  upon  the  concentra- 
tion of  the  soluble  constituents  in  the  solid  phases  in  other  than 
definite  chemical  combinations.  Other  factors  affecting  the  con- 
centration of  the  mineral  constituents  in  the  soil  solution  are 
always  existent,  and  theoretically  at  least,  can  not  be  ignored. 
Nevertheless  a  priori  reasoning  as  well  as  the  experimental 
evidence  at  hand  indicates  that  the  various  processes  taking  place 
in  the  soil  as  a  whole  continually  tend  to  form  and  maintain  a 
normal  concentration  of  mineral  constituents  in  the  soil  solution. 


Chapter  VIII. 

ABSORPTION  BY  SOILS. 

A  property  of  soils,  affecting  profoundly  the  composition  and 
concentration  of  the  soil  solution,  is  absorption.1  It  is  generally 
recognized  that  soils  are  good  absorbers  for  vapors,  and  this 
fact  finds  practical  expression  in  the  common  practice  of  bury- 
ing things  with  a  disagreeable  odor,  such  as  animal  carcasses, 
night-soil,  etc.  It  is  also  well-known  that  dissolved  as  well  as 
suspended  material  can  be  more  or  less  completely  removed  from 
water  by  passing  it  through  sand  or  soil,  and  this  fact  finds 
important  application  in  water  supplies  for  cities  and  towns, 
sewage  disposal,  etc.  It  was  known  as  long  ago  as  Aristotle's 
time  that  ordinary  salt  is  partly  removed  from  water  by  passing 
through  sand  or  soil.  In  recent  times  the  practical  as  well  as 
theoretical  importance  of  this  phenomenon  has  led  to  consider- 
able study  and  experimental  research,  so  that  our  knowledge  of 
absorption  effects  is  now  fairly  extensive,  though  it  can  hardly 
be  claimed  that  it  is  satisfactory.  The  absorption  of  a  dissolved 
substance  from  solution  by  a  soil  may  be  one  or  more  of  at  least 
three  kinds  of  phenomena.  It  may  be  a  mechanical  inclusion  or 
trapping,  distinguished  by  the  term  imbibition,  the  most  familiar 
and  striking  case  being  the  absorption  of  water  itself  by  soil  or 
sponge  or  similar  medium.  It  may  be  a  partial  taking  up  of 
the  dissolved  substance  to  form  a  new  compound  or  a  solid 
solution,2  as  probably  is  the  absorption  of  phosphoric  acid  by 

1  For  a  detailed  discussion  and  citations  of  the  literature,  see :   Absorp- 
tion of  vapors  and  gases  by  soils,  by  H.  E.  Patten  and  F.  E.  Gallagher, 
Bull.  No.  51 ;  and  Absorption  by  soils,  by  H.  E.  Patten  and  W.  H.  Wagga- 
man,  Bull.  No.  52,  Bureau  of  Soils,  U.  S.  Dept.  Agriculture,  1908. 

2  That  is,  a  homogeneous  solid,  which  may  be  either  crystalline  or 
amorphous.     Probably  the   readiest  criterion   for  distinguishing  between 
a  definite  compound  and  a  solid  solution,  is  that  the  former  is  stable  in 
contact  with  a  liquid  solution  of  its  constituents  over  a  measurable  range 
of  concentrations,  while  the  composition   of  the   solid   solution  changes 
with  every  change  in  the  concentration  of  the  liquid  solution  in  contact 
with  it. 


60  THE:  soil,  SOLUTION 

lime  or  ferric  oxide.  Or  it  may  be  a  condensation  or  concentra- 
tion of  the  dissolved  substance  on  or  about  the  surface  of  the 
absorbing  medium,  a  phenomenon  known  as  adsorption.  To 
prove  the  existence  of  adsorption  definitely  and  conclusively  in 
any  given  case  is  always  difficult,  if  ever  possible,  but  the  exist- 
ence of  this  phenomenon  is  the  most  logical  explanation  of  many 
observations,  and  is  generally  admitted  by  chemists  and  physicists 
at  the  present  time.1  It  is  by  adsorption,  probably,  that  potash 
and  ammonia  are  held  by  the  soil  when  added  in  fertilizers. 

That  absorption  is  dependent  in  some  manner  upon  the  solu- 
bility of  the  dissolved  substance  in  the  particular  solvent  em- 
ployed would  seem  to  be  obvious.  But  what  the  relation  may 
be,  if  it  exists  at  all,  is  not  known.  For  instance,  silk  absorbs 
picric  acid  from  solutions  in  water  and  alcohol  but  not  from  solu- 
tions in  benzene,  although  the  solubility  of  picric  acid  in  benzene 
lies  between  its  solubility  in  water  and  in  alcohol.2 

The  absorption  of  any  given  dissolved  substance  from  differ- 
ent solvents  is  markedly  different.  Most  soils  absorb  methylene 
blue  from  aqueous  solutions  with  great  avidity,  but  washing  out 
the  absorbed  dye  with  water  is  an  extremely  tedious  and  unsatis- 
factory process,  although  the  dye  can  be  readily  and  more  or 
less  completely  removed  from  the  soil  by  alcohol.  As  might  be 
anticipated  from  this,  it  is  known  that  the  presence  of  one  dis- 
solved substance  affects  the  absorption  of  another,  but  in  what 
way  can  not,  generally,  be  anticipated,  although  it  would  seem 
that  the  importance  of  this  subject  for  manurial  practice  would 
invite  further  research. 

From  the  same  solution,  different  absorbents  remove  a  dis- 

1 A  clear  and  apparently  indisputable  case  of  adsorption  has  been 
noted  by  Patten  (Some  surface  factors  affecting  distribution,  Trans.  Am. 
Electrochem.  Soc.,  10,  67-74  (1906).  On  adding  powdered  quartz  to  an 
aqueous  solution  of  gentian  violet,  there  is  a  distribution  of  the  dye 
between  the  water  and  the  quartz.  A  microscopic  examination  of  the 
latter  showed  that  the  dye  was  concentrated  in  thin  layers  upon  the  sur- 
face of  the  quartz  grains,  from  which  it  could  be  washed  with  water,  no 
change  in  the  quartz  grains  being  noticeable. 

2  Absorption  of  dilute  acids  by  silk,  by  James  Walker  and  James  R. 
Appleyard,  Jour.  Chem.  Soc.,  69,  1334-1349  (1896). 


ABSORPTION    BY   SOILS  6l 

solved  substance  in  different  degrees.  Speaking  generally,  paper 
absorbs  dyes  more  readily  than  do  soils,  while  soils  absorb  bases 
more  readily  than  does  paper.  Hence  the  reddening  of  litmus 
paper  when  in  contact  with  a  moist  soil.  Heavy  soils  or  soils 
containing  much  hydrated  ferric  oxide  absorb  bases  more  readily 
than  do  light  soils,  but  this  is  probably  owing  to  relative  amounts 
of  surface  exposed,  for  the  same  relation  holds  true  with  r"espect 
to  phosphoric  acid.  Soils  rich  in  humus  are  better  absorbers 
than  soils  not  so  rich.  But  here  again  there  is  yet  doubt  as  to 
whether  the  explanation  lies  in  the  amount  or  in  the  kind  of 
surface  acting. 

From  the  same  solvent  different  dissolved  substances  are 
absorbed  quite  differently  by  any  given  absorbent.  This  can  be 
readily  illustrated  again  by  dyes.  If  an  aqueous  solution  of  a 
mixture  of  methylene  blue  and  sodium  cosine,  for  instance,  be 
shaken  up  with  a  soil,  or  percolated  through  a  column  of  soil, 
the  methylene  blue  is  absorbed  the  more  quickly  and  completely 
and  a  partial  separation  of  the  two  dyes  can  be  readily  effected, 
the  separation  being  more  or  less  complete  according  to  the 
conditions  of  the  experiment.  In  the  same  manner  two  salts  in 
solution  can  be  separated  partially  at  least.1  Soils  absorb  potas- 
sium more  readily  than  sodium;  magnesium  more  readily  than 
lime;  and  ammonia  more  readily  than  any  of  these  bases.2 

The  absorption  from  aqueous  solutions  of  inorganic  salts 
involves  a  most  interesting  complication.  Just  as  a  mixture  of 
two  or  more  dyes  or  salts  in  solution  can  be  separated  by  the 
selective  action  of  an  absorbent,  so  can  an  electrolyte  itself  be 
decomposed  or  resolved.  Thus,  if  a  solution  of  potassium 

1  For  a  number  of  interesting  examples,  see,  Ueber  das  Auf steigen 
von  Salzlosungen  in  Filtrirpapier,  von  Emil  Fischer  und  Edward  Schmid- 
mer,  Liebig's  Annalen  der  Chemie,  272,  156-169  (1893). 

1  The  prompt  absorption  of  a  base  by  soils  is  shown  by  the  following 
experiment :  To  some  freshly  boiled  distilled  water  add  several  drops 
of  alcoholic  phenolphthalein,  and  then  just  enough  base  to  produce  a 
decided  red  color.  If  the  solution  be  now  passed  through  a  short  column 
of  soil,  cotton,  shredded  filter-paper  or  similar  absorbent,  the  percolate 
will  be  perfectly  colorless.  The  red  color  will  be  restored,  however,  by 
adding  a  little  of  the  base  to  the  percolate. 


62  THE   SOIL   SOLUTION 

chloride  be  passed  through  a  column  of  soil,  or  cotton,  or  paper, 
or  any  similar  absorbent,  the  filtrate  will  not  only  be  less  con- 
centrated than  the  original  solution,  but  the  potassium  will  be 
found  to  have  been  absorbed  to  a  greater  extent  than  the 
chlorine,  that  is,  the  percolate  contains  free  hydrochloric  acid. 
The  importance  of  this  phenomenon  for  the  conservation  of  the 
desirable  constituents  of  manurial  salts,  and  the  elimination  or 
leaching  out  of  the  less  desirable  constituents  is  obviously  great. 
Equally  great  perhaps,  is  the  effect  upon  the  reaction  of  the 
soil,  whether  it  be  rendered  temporarily  alkaline  or  acid,  an 
effect  of  the  very  greatest  importance  for  the  growth  of  some 
of  our  common  crop  plants1  and  for  the  lower  soil  organisms, 
such  as  the  bacteria,  molds,  together  with  ferments,  enzymes, 
etc.,  many  of  which  are  very  sensitive  to  the  reaction  of  the 
media  in  which  they  may  be,  and  which  in  turn  are  of  undoubted 
importance  in  determining  the  fertility  of  the  soil  for  higher 
plants. 

The  absorption  of  a  dissolved  substance  from  solution  by  an 
absorbent  is  in  effect  a  distribution  phenomenon  and  the  simplest 
formula  to  give  quantitative  expression  to  such  a  distribution  is 
C/C1  =  K  when  C  is  the  concentration  in  the  liquid  phase  and 
C1  the  concentration  in  the  solid  phase,  K  being  a  characteristic 
constant  for  the  particular  case  under  consideration.  When  a 
chemical  reaction  or  a  change  of  state,  chemical  or  physical,  is 
involved  in  the  absorption  in  either  dissolved  substance  or  absor- 
bent the  formula  becomes  O/C1  =  K  when  n  is  a  function  which 
may  be  very  simple  or  very  complex.  Attempts  to  develop  a 
precise  formula  of  this  general  type  for  the  absorption  by  some 
given  soil,  although  such  a  formula  would  be  desirable  for 

1  See,  The  toxic  action  of  acids  and  salts  on  seedlings,  by  F.  K. 
Cameron  and  J.  F.  Breazeale,  Jour.  Phys.  Chem.,  8,  1-13  (1904).  It  is 
quite  conceivable,  for  instance,  that  if  the  drainage  conditions  were  not 
exceptionally  good  under  a  heavy  type  of  soil,  it  might  be  rendered 
temporarily  unfit  for  clover  or  alfalfa  by  a  heavy  application  of  potas- 
sium salts  or  of  sodium  nitrate.  The  idea  put  forward  by  some  authori- 
ties that  too  long  continued  or  over  fertilizing  renders  soils  acid,  may 
have  better  foundation  than  their  theoretical  reasoning  would  seem  to 
warrant. 


ABSORPTION   BY   SOILS  63 

theoretical  and  practical  reasons  alike,  have  uniformly  failed. 
A  sufficient  reason  for  this  failure  seems  to  lie  in  the  fact  that 
most  dissolved  substances  produce  an  appreciable  effect  on  the 
granulation  or  flocculation  of  the  soil  particles,  which  is  pro- 
gressive with  the  absorption  so  that  a  continual  change  of  absorb- 
ing or  effective  surface  is  taking  place  as  the  absorption  proceeds.1 
Moreover,  in  the  case  of  an  absorption,  with  the  formation  of  a 
continuous  film  of  the  dissolved  substance,  a  new  kind  of  absorb- 
ing surface  is  developed.  Hence  n  is  a  function  of  so  difficult 
a  character  as  to  defy  thus  far  any  attempt  at  formulation.2 

We  cannot  therefore  predict  in  any  quantitative  way  what 
will  be  the  distribution  of  a  soluble  substance  such  as  salts  in 
commercial  fertilizers,  for  instance,  between  the  solid  soil 
particles  and  the  soil  solution.  Empirical  experiments  show, 
however,  that  with  the  amount  of  a  soluble  salt  present  under 
normal  conditions  in  a  humid  climate,  or  as  used  in  fertilizer 
practice,  the  absorption  of  ammonia,  lime,  potassium  or  phos- 
phoric acid  is  relatively  very  great,  and  in  a  general  way  in  about 
the  order  named. 

Absorption  is  not  an  instantaneous  process.  However,  the 
rate  at  which  a  dissolved  substance  is  absorbed  is  generally  quite 
rapid.  That  is,  if  a  soil  be  stirred  or  mixed  with  an  aqueous 
solution,  the  absorption  takes  place  very  quickly,  in  the  absence 

1  That  mineral  fertilizers  have  a  decided  influence  on  the  granula- 
tion of  soils  and  the  properties  dependent  thereon,  and  that  this  is  of 
practical   importance,    is    gradually    coming   to    be    recognized;     see,    for 
instance,  Ein  Beitrag  zur  Kenntnis  der  Wirkung  kimstlicher  Diinger  auf 
die  Durchlassigkeit  des  Bodens  fur  Wasser,  von  Edwin  Blanck,  Landw. 
Jahrb.,  38,  863-869   (1909),  and  the  literature  there  cited.     Dr.  R.  O.  E. 
Davis  in  a  yet  unpublished  investigation  has  shown  that  the  addition  of 
soluble   salts   produces   decided    effects   upon   the    soil-moisture    relations 
which  affect  crop  production.     The  critical  moisture  content  is  displaced, 
the   penetrability,   permeability,   specific   volume,   vapor   tension,   etc.,    are 
affected  in  measurable  degree,  and  it  appears  that  the  physical  functions 
of  mineral  fertilizers  are  much  greater  in  amount  and  importance  than 
has  been  popularly  assumed. 

2  The  distribution  of  solute  between  water  and  soil,  by  F.  K.  Cameron 
and  H.  E.  Patten,  Jour.  Phys.  Chem.,  11,  581-593  (1907). 


64  THE   SOIL   SOLUTION 

of  any  outside  disturbing  influences.  The  law  governing  the 
rate  of  absorption  by  soils  has  not  therefore  possessed  any  great 
practical  interest  and  has  not  been  studied  from  a  quantitative 
point  of  view,  although  it  is  known  qualitatively  that  the  rate 
is  increased  by  increasing  the  concentration  of  the  solution, 
or  by  increasing  the  amount  of  the  absorbent  or  at  least  its 
effective  surface.  Two  rate  equations  are  of  interest  in  this  con- 
nection, and  have  been  carefully  studied.  The  rate  at  which  a 
salt  or  other  dissolved  substance  will  advance  into  an  absorbing 
soil  from  a  solution  is  given  by  the  same  equation  as  that  describ- 
ing the  rate  of  advance  of  the  water  itself,  yn  =  kt  where  y  is 
the  distance  and  t  the  time.1  The  constants  n  and  k  for  the 
slower  moving  dissolved  substance  are  different  from  those  for 
the  water.  This  equation  has  probably  little  importance  for 
ordinary  agriculture,  for  absorption  by  the  soil  from  a  large  (and 
relatively  illimitable)  mass  of  solution  is  unusual.  That  it  may 
have  considerable  importance  in  seepage,  irrigation,  and  some 
soil  engineering  problems,  seems  quite  likely. 

The  rate  at  which  a  soil  will  absorb  a  dissolved  substance  from 
a  percolating  solution  is  given  by  the  equation  dx/dt  =  K(A — 
x},  as  has  been  pointed  out  above.2  More  interesting  and 
important,  however,  is  the  fact  that  this  same  equation  describes 
the  rate  at  which  an  absorbed  substance  is  removed  from  the 
soil  by  leaching.  In  the  case  of  soils  in  humid  areas  dx/dt 
rapidly  becomes  exceedingly  small  as  x  approaches  A,  that  is, 
when  the  amount  of  soluble  material  in  the  soil  becomes  small, 
and  is  practically  constant  under  such  conditions,  as  has  been 
pointed  out  above  when  describing  the  removal  of  potassium 
and  phosphoric  acid  from  soils  by  percolating  waters.  This 
formula  has  a  special  interest  in  considering  the  reclamation  of 
alkali  lands  by  underdrainage,  a  problem  to  which  reference 
will  be  made  later. 

Both  percolation  experiments,  as  those  cited  above,  and  direct 
absorption  experiments  made  by  shaking  up  soils  with  solutions 

1  See  formula,  page  28. 
*  See  formula,  page  47. 


ABSORPTION    BY   SOILS  65 

of  the  salts  in  question,  show  conclusively  that  the  absorption 
phenomena  taking  place  in  the  soil  are  in  harmony  with  the  direct 
solubility  effects  in  tending  to  produce  and  maintain  a  solution 
of  a  normal  concentration  as  regards  those  constituents  which 
it  happens  are  also  derived  from  the  soil  minerals.1  It  is  an 
interesting  coincidence  that  nitric  acid  (in  combination  with 
various  bases  of  course)  is  very  little  absorbed  by  most  soils,  and 
does  vary  in  concentration,  not  only  in  different  soils  but  in  the 
same  soil,  between  wide  limits,  and  within  short  intervals  of 
time.2  The  nitrates  of  the  soil  are  not  derived  from  minerals, 
and  should  more  properly  be  considered  with  the  organic  con- 
stituents of  the  soil  solution. 

An  important  application  of  these  views  concerning  absorp- 
tion arises  in  connection  with  certain  widespread  notions  con- 
cerning soil  acidity.  There  is  a  popular  belief  that  most  soils 
are  acid,  that  the  soil  solution  contains  some  free  acid,  mineral 
or  organic,  other  than  dissolved  carbon  dioxide,  and  that  a 
neutral  or  alkaline  solution  is  necessary  to  the  successful  pro- 
duction of  most  of  our  crops.  This  belief  is,  however,  unwar- 
ranted, for  the  vast  majority  of  soils  yield  an  aqueous  extract 
which  is  alkaline  when  boiled  to  expel  carbon  dioxide,  and  some 
of  our  crops,  for  instance  wheat,  seem  to  thrive  better  in  a 
slightly  acid  medium.  This  popular  fallacy  seems  to  have  its 

*An  extreme  case  is  worth  citing  in  this  connection.  Mr.  W.  H. 
Heileman  in  studying  the  influence  of  various  kinds  of  alkali  upon  plant 
growth,  added  from  3-4  per  cent,  of  sodium  carbonate  to  soils  known  to 
be  otherwise  free  from  alkali.  Wheat  seedlings  grown  in  the  soils  so 
treated  showed  no  ill  effects  from  the  added  salt.  When  distilled  water 
was  percolated  slowly  through  the  soils,  or  shaken  up  w'th  them,  the 
resulting  solution  contained  the  merest  traces  of  the  alkali. 

The  ordinary  method  of  determining  the  lime  requirement  of  a  soil 
by  adding  lime  water  until  the  solution  shows  an  alkaline  reaction,  is 
another  obvious  absorption  phenomenon,  and  is  not  dependent,  as  popu- 
larly supposed,  upon  the  presence  of  acids  in  the  soil.  Soils  which  by 
no  possibility  could  contain  any  free  acid,  frequently  absorb  very  large 
amounts  of  lime  in  this  manner. 

2  Usually,  in  the  growing  season,  the  soil  solution  has  a  much  higher 
concentration  with  respect  to  nitrates  in  the  morning  than  it  has  in  the 
evening. 


66  THE  SOIL   SOLUTION 

origin  in  the  fact  that  most  soils  when  moistened  and  pressed 
against  blue  litmus  paper,  redden  it.  This  reddening  may  some- 
times be  due  to  the  actual  presence  of  some  acid,  or  to  dissolved 
carbon  dioxide,  but  is  undoubtedly  due  in  the  majority  of  cases 
to  selective  absorption.  Litmus  is  a  red  dye  of  an  acid-like 
character,  which  forms  a  soluble  blue  salt  with  the  ordinary 
bases.  But  it  has  been  shown  that  most  soils  are  better  absorb- 
ents of  bases  than  is  paper,  whereas  paper  is  a  better  absorbent 
of  dye,  speaking  generally,  than  is  a  soil.  Consequently  when 
moist  soil  is  brought  into  contact  with  wetted  blue  litmus  paper 
the  base  is  absorbed  more  readily  by  the  soil,  and  the  dye  by  the 
paper,  the  latter  therefore  becoming  reddened. 

The  reddening  of  blue  or  "neutral"  litmus  paper  can  be  accom- 
plished with  various  absorbents.  By  pressing  the  litmus  paper 
between  moistened  wads  of  absorbent  cotton  the  reddening  can 
be  readily  accomplishe,d  usually  in  the  course  of  ten  minutes 
to  a  half  hour.  That  the  phenomenon  is  not  due  to  any  adhering 
acid  on  the  cotton  can  be  shown  in  the  following  way:  A  litmus 
solution  is  carefully  prepared  so  that  there  is  a  very  small  excess 
of  base  present  over  that  required  to  give  the  blue  color.  A 
wad  of  absorbent  cotton  is  carefully  washed  by  repeatedly  sous- 
ing it  in  distilled  water  from  which  carbon  dioxide  has  been 
expelled  by  boiling.  When  the  cotton  has  been  thoroughly 
washed,  it  is  stirred  thoroughly  in  a  portion  of  distilled  water, 
free  from  carbon  dioxide,  then  withdrawn  by  some  appropriate 
instrument  and  allowed  to  drain  for  a  few  minutes.  The  litmus 
is  added  in  fairly  large  quantity  to  the  drainings,  which  should 
then  have  a  blue  color.  Again  stir  the  cotton  in  the  water,  and 
more  or  less  quickly,  depending  on  the  amount  and  purity  of 
the  litmus  preparation  as  well  as  the  quantity  of  cotton  used, 
the  solution  will  become  red.  The  only  criterion  for  deter- 
mining surely  that  a  soil  is  acid,  is  to  make  an  aqueous  extract, 
expel  the  dissolved  carbon  dioxide  by  boiling,  or  by  passing 
through  the  solution  an  inactive  gas,  such  as  nitrogen,  and  then 
to  test  the  reaction  of  the  solution.  Acid  soils  undoubtedly  do 
exist,  but  they  are  by  no  means  common  or  wide-spread,  and 
are  to  be  regarded  as  exceptional  and  abnormal. 


ABSORPTION   BY   SOILS  67 

The  phenomena  of  selective  absorption  suggest  the  important 
part  which  surfaces  play  in  modifying  and  changing  chemical 
reactions.1  For  instance,  Becquerel2  observed  that  a  solution  of 
copper  nitrate  or  cobalt  chloride  diffusing  from  a  cracked  test- 
tube  placed  in  a  solution  of  sodium  sulphide,  led  to  the  form- 
ation of  the  corresponding  sulphide,  but  in  the  crack  the  metal 
itself  was  precipitated.  Experiments  of  Graham3  show  that 
when  a  solution  of  silver  nitrate  is  percolated  through  charcoal, 
not  only  is  there  a  selective  absorption  as  is  shown  by  the  perco- 
late containing  free  acid,  but  there  is  a  chemical  reaction  involved, 
since  the  silver  is  deposited  in  metallic  spangles  in  the  interstices 
of  the  absorbent.  Graham  has  shown,  and  since  his  time  others, 
that  often  metals  can  be  separated  from  solutions  of  their  salts 
by  such  absorbents  as  charcoal.  Spring4  has  shown  that  at 
bounding  surfaces  of  dilute  solutions,  chemical  action  is 
increased. 

It  has  been  shown  that  the  amount  and  kind  of  surface  has  a 
marked  influence  on  the  decomposition  of  hypochlorous  acid, 
carbon  dioxide,  phosphine,  arsine,  and  other  compounds.  Meyer 
and  his  associates,  as  well  as  a  number  of  other  investigators, 
have  shown  that  the  character  of  the  surface  of  the  containing 
vessel  greatly  affects  the  combination  of  hydrogen  and  oxygen. 
Many  reactions  have  been  investigated  by  van't  Hoff,  who  con- 
cludes that  both  the  nature  and  amount  of  surface  exposed  have 
an  influence.  The  inversion  of  sugar  is  affected  by  the  nature 
of  the  walls  of  the  containing  vessel,  and  its  reduction  by  Fehl- 
ing's  solution  is  affected  both  by  the  walls  of  the  vessel  and  the 
amount  of  cuprous  oxide  formed  in  the  reaction.  Alteration  in 
the  character  as  well  as  degree  of  a  number  of  reactions  by  having 
them  take  place  in  capillary  spaces  has  been  observed  by 

1  For  references  to  the  literature  see,  Bull.  No.  30,  Bureau  of  Soils, 
U.  S.  Dept.  Agriculture,  p.  61  et  seq. 

2  Note  sur  les  reductions  metalliques  produites  dans  les  espaces  capil 
laires,  par  M.  Becquerel,  Comptes  rendus,  82,  354-356  (1876). 

3  Effects  of  animal  charcoal  on  solutions,  by  T.  Graham,  Quart.  Jour. 
Sci.,  1,  120-125  (1830). 

4  Uber   eine  Zunahme  chemischer   Energie  an  der   f reien  Oberflache 
fliissiger  Korper,  von  W.  Spring,  Zeit.  physik.  Chem.,  4,  658-662  (li 


68  THE   SOII,   SOLUTION 

Liebreich,  Becquerel,  Lieving  and  other  investigators.  So-called 
"contact  reactions,"  as  in  the  production  of  sulphuric  acid,  are 
now  familiar  processes  rinding  commercial  applications.  And 
the  solubility  of  some  substances  at  least,  notably  gypsum,  has 
been  shown  to  vary  considerably  with  the  size  and  consequent 
shape  of  the  particles  in  the  solid  substance  in  contact  with  its 
solution.1 

It  has  been  shown  that  some  soils  will  at  times  produce  the 
blue  coloration  in  alcoholic  solutions  of  guiac,  which  is  char- 
acteristic of  oxidases,  and  yellow  aloin  solutions  are  sometimes 
colored  red.  Hydrogen  peroxide  is  decomposed  by  some  soils 
even  after  they  have  been  thoroughly  ignited  to  get  rid  of  all 
organic  matter.  But  in  how  far  these  effects  may  be  due  to 
surface  influences  can  not  be  positively  stated;  yet  uncompleted 
investigations  by  Dr.  M.  X.  Sullivan  indicate  that  some  of  these 
phenomena  at  least  must  be  attributed  to  specific  influences 
(although  probably  of  catalytic  character)  of  particular  soil 
components,  such  possibly  as  manganous  oxide  or  ferric  oxide; 
but  the  mechanism  of  the  reactions  is  as  yet  largely  speculative. 

The  soil  is  composed  in  large  part  of  very  fine  particles  of 
rounded  shape,  exposing  relatively  an  enormous  surface  to  the 
soil  solution,  and  normally  this  solution  is  mainly  under  capillary 
conditions,  so  that  we  should  expect  that  many  reactions  would 
take  place  quite  differently  in  the  soil  from  the  way  they  would 
in  a  beaker  or  flask.  This  fact  has  been  generally  overlooked 
or  ignored,  and  is  probably  the  explanation  of  many  of  the 
apparently  anomalous  results  hitherto  reported  in  chemical  invest- 
igations of  soils.  Abnormal  solubilities,  precipitations,  oxida- 
tions or  reductions  are  frequently  found  in  the  literature,  and 
when  their  abnormality  is  noted  at  all,  they  are  too  often  and 
with  slight  show  of  reason  ascribed  to  indefinite  bacterial  action 
or  more  mysterious  vital  agencies.  Many  of  them  are  undoubt- 
edly the  results  of  surface  actions.  Unfortunately,  aside  from 
1  See  especially,  Beziehungen  swischen  Oberflachenspannung  und  Los- 
lichkeit,  von  G.  A.  Hulett,  Zeit.  Phys.  Chem.,  37,  385-406  (1901)  ;  Loslich- 
keit  und  Loslichkeits  Beeinflussung,  von  V.  Rothmund,  p.  109  (1007)  ; 
Principles  theoretiques  des  methodes  d'analyse  minerale,  par  G.  Chesneau, 
pp.  16-25  (1906). 


-       ABSORPTION    BY   SOILS  69 

some  few  studies  of  absorption  phenomena,  surface  effects  have 
received  little  or  no  attention  from  soil  investigators,  although 
obviously  one  of  the  most  important  and  apparently  fruitful 
fields,  requiring  immediate  attention.  Enough  is  known  to 
justify  the  statement  that  the  chemistry  of  the  soil  need  not  be, 
and  probably  is  not,  the  chemistry  of  the  beaker. 


Chapter  IX. 

THE  RELATION  OF  PLANT  GROWTH  TO  CONCENTRATION. 

That  the  concentration  of  the  mineral  constituents  in  the  soil 
solution  under  normal  conditions  is  competent  for  plant  support, 
is  shown  by  numerous  experiments.  Birner  and  Lucanus1  in  an 
experiment  that  has  long  since  become  classic,  found  that  they 
could  raise  wheat  to  maturity  in  a  well-water,  the  concentration 
of  which  was  approximately  18  parts  per  million  with  respect 
to  potassium,  and  2  parts  per  million  with  respect  to  phosphoric 
acid,  while  the  corresponding  concentrations  of  the  soil  solution 
are  normally  about  25-30  parts  per  million  of  potassium  and 
6-8  parts  per  million  of  phosphoric  acid.  Nevertheless  Birner 
and  Lucanus  report  that  the  wheat  grown  in  the  well-water 
throve  even  better  than  that  grown  at  the  same  time  in  a  rich 
garden  mold.  Since  then  many  investigators  in  numerous  trials 
have  obtained  similar  results.  Recently  wheat,  corn,  and  some 
of  the  common  grasses  have  been  grown  to  a  satisfactory  matur- 
ity in  tap  water  with  a  concentration  of  about  7  parts  per  million 
of  potassium  and  0.5  parts  per  million  of  phosphoric  acid.  And 
repeatedly  wheat  plants,  grasses,  cowpeas,  vetches,  potatoes  and 
other  plants  have  grown  in  a  satisfactory  way  in  solutions  made 
by  shaking  up  a  soil  in  distilled  water  and  separating  from  the 
solid  particles  by  means  of  filters  of  unglazed  porcelain. 

There  can  be  no  doubt,  therefore,  that  the  soil  solution  is 
normally  of  a  concentration  amply  sufficient  to  support  ordinary 
crop  plants,  and  is  maintained  at  a  sufficient  concentration,  so 
far  as  mineral  plant  nutrients  are  concerned.  Undoubtedly, 
however,  variations  in  the  concentration  of  the  soil  solution  can, 
and  often  do,  take  place,  and  the  results  of  laboratory  experiment 
indicate  that  they  probably  produce  effects  on  plants. 

It  has  been  shown  in  water-culture  experiments  with  wheat, 
that  if  a  given  ratio  of  mineral  nutrients  be  maintained,  relatively 
small  effect  is  produced  on  the  growing  plants  by  varying  the 

1  Wasserculturversuche  mit  Haf er,  von  Dr.  Birner  und  Dr.  Lucanus, 
Landw.  Vers.-Sta.,  8,  128-177  (1866). 


RELATION    OF   PLANT  GROWTH   TO   CONCENTRATION  7 1 

concentration  over  a  wide  range,  in  one  case  from  75  parts  per 
million  to  750  parts  per  million,1  and  this  effect  seems  to  be 
largely  independent  of  the  nature  of  the  particular  mixture  of 
solutes.  But  varying  the  relative  proportions  of  the  mineral 
constituents  has  been  shown  by  numerous  experiments  to  produce 
very  marked  changes  in  the  growth  of  plants.  Not  only  does  a 
control  of  the  concentration  and  proportion  of  the  mineral  con- 
stituents of  a  solution  produce  a  more  rapid,  or  a  slower  growth, 
a  greater  or  lesser  total  growth,  but  it  produces  differences  in 
the  character  of  growth;  as  for  instance,  causing  the  tops  to 
grow  relatively  faster  than  the  roots,  or  vice  versa.  However, 
many  effects  of  this  type  can  be  produced,  and  sometimes  more 
readily,  by  soluble  organic  substances,  or  mechanical  agencies. 
The  mechanism  of  these  effects  is  by  no  means  clear,  in  many 
cases.  That  other  causes  obtain  than  a  sufficient  supply  of 
mineral  nutrients  will  be  shown  in  the  following  chapters.  Ex- 
periments with  wheat  seedlings  in  water  cultures,  where  the 
weights  of  the  green  tops  were  taken  as  the  measure  of  growth, 
showed  that  the  most  favorable  ratio  was  one  of  phosphoric 
acid  (PO4)  to  three  or  four  of  potassium  (K),  about  the  ratio 
which  has  been  found  to  exist  normally  in  the  soil  solution  of 
humid  areas  of  the  United  States,  namely,  6-8  parts  per  million 
of  phosphoric  acid  to  25-30  parts  per  million  of  potassium. 

All  growing  plants  require  for  their  growth  and  development 
various  organic  compounds  containing  carbon,  hydrogen,  oxygen 
and  nitrogen.  The  higher  crop  plants  with  which  agricultural 
investigations  appear  to  be  more  immediately  concerned,  seem 
to  have  inherent  power  to  produce  these  needed  substances 
within  themselves.  But  it  is  becoming  more  and  more  evident 
that  the  large  problem  of  soil  fertility,  or  the  relation  of  the 
soil  to  crop  production,  frequently  if  not  generally  involves  the 
growth  and  development  of  lower  organisms  including  ferments 
and  bacteria.  These  may  or  may  not  in  particular  cases,  favor 
the  growth  of  the  desired  higher  plants.  Many  of  these  lower 
organisms  require  certain  organic  compounds  or  thrive  better 

1  Effect   of   the   concentration   of   the    nutrient    solution   upon    wheat 
cultures,  by  J.  F.  Breazeale,  Science,  n.  s.,  22,  146-149  (1905). 


72  THE  SOIi,   SOLUTION 

if  these  are  brought  to  them  in  the  soil  solution,  and  indeed 
evidence  is  not  lacking  that  such  may  sometimes  be  the  case 
even  with  the  higher  plants.  Certainly  their  growth  can  be 
much  affected  by  the  presence  of  different  organic  substances  in 
the  nutrient  solution.  Enough  work  has  been  done  in  this  field 
of  investigation  to  show  that  the  concentration  of  the  soil  solution 
or  artificial  nutrient  solution  with  respect  to  the  organic  com- 
pounds must  generally  be  low;  too  high  a  concentration  always 
inhibits  growth  or  even  produces  death;  and  there  is  probably 
an  optimum  concentration,  or  one  at  which  the  plant  will  grow 
best;  but  this  optimum  concentration  varies  with  the  specific 
nature  of  the  plant,  the  presence  of  other  dissolved  substances, 
mineral  or  organic,  and  possibly  with  other  factors.  While  a 
notable  amount  of  work  has  thus  been  done  in  a  field  of  inquiry 
obviously  of  practical  as  well  as  theoretical  interest,  almost  no 
definite  information  has  as  yet  been  obtained  as  to  the  concen- 
tration of  organic  substances  in  the  soil  solution,  or  its  effect 
upon  plants  under  field  conditions,  excepting  in  the  case  of  the 
nitrates,  the  products  of  bacterial  activities.  The  concentration 
with  respect  to  nitrates  is  known  to  vary  greatly  from  a  few 
parts  to  several  thousand  parts  per  million,  and  this  sometimes 
within  a  few  days  or  even  hours.  The  great  changes  in  con- 
centration with  respect  to  nitrates,  the  rapidity  of  the  changes, 
and  the  correspondingly  large  effects  on  growing  plants  make 
this  a  subject  requiring  special  treatment  by  itself.  This  at 
present  seems  more  easily  appreciated  from  a  consideration  of 
the  bacteria  involved,  and  will  not  be  discussed  more  fully  here.1 
Of  the  ash  constituents  of  plants,  there  must  be  in  the  soil 
solution,  potassium,  magnesium,  phosphorus,  sulphur  and  iron 
for  any  plant  growth,  and  for  the  higher  crop  plants,  calcium 
1  See :  The  fixation  of  atmospheric  nitrogen  by  bacteria,  by  J.  G. 
Lipman,  Bull.  No.  81,  Bureau  of  Chemistry,  U.  S.  Dept.  of  Agriculture, 
1904;  A  review  of  investigations  in  soil  bacteriology,  by  Edward  B. 
Voorhees  and  Jacob  G.  Lipman,  Bull.  No.  194,  Office  of  Experiment  Sta- 
tions, U.  S.  Dept.  of  Agriculture,  1907;  The  physiology  of  plants,  by 
W.  Pfeffer,  translated  by  A.  J.  Ewart,  vol.  I,  p.  388  et  seq.,  1900;  The 
effect  of  partial  sterilization  of  soil  on  the  production  of  plant  food,  by 
Edward  John  Russell  and  Henry  Brougham  Hutchinson,  Jour.  Agric. 
Sci.,  3,  111-144  (1909). 


RELATION   OF   PLANT  GROWTH   TO   CONCENTRATION  73 

must  also  be  present.  Of  these,  iron  is  usually  present  in  barely 
appreciable  concentration  and  more  than  this  is  not  desirable, 
or  is  even  harmful  for  common  crop  plants.  Under  the  normal 
conditions  for  soils  in  humid  areas,  sulphur  also  is  usually  present 
in  scarcely  more  than  appreciable  quantities  and  there  is  no 
positive  evidence  to  show  that  higher  concentrations  are  especially 
desirable,  though  this  may  be  the  case  for  certain  crops,  such  for 
instance  as  the  onion.  Phosphorus  is  usually  present  to  the 
extent  of  5  or  6  parts  per  million  of  phosphoric  acid  (P2O5), 
while  it  has  repeatedly  been  shown  that  such  crops  as  wheat 
can  thrive  and  make  a  good  growth  with  a  concentration  a 
tenth  of  this.  It  appears  to  be  clear  therefore  that  as  far  as 
food  supply  is  concerned  there  is  normally  an  ample  supply  of 
phosphorus  in  the  soil  solution;  but  it  does  not  follow  that 
increasing  the  concentration  of  the  solution  if  only  temporarily 
would  not  result  in  favorable  effects  upon  growing  plants. 

A  consideration  of  the  bases,  however,  introduces  serious  diffi- 
culties, which  will  probably  require  much  further  research  by 
the  plant  physiologist  as  well  as  the  soil  chemist.  It  is  impossible 
as  yet  to  determine  the  concentrations  at  which  different  plants 
will  not  grow.  It  is  even  impossible  to  determine  the  concentra- 
tions at  which  they  will  thrive  best.  It  seems  certain  that  differ- 
ent crop  plants  require  different  amounts  of  these  minerals,  but 
whether  or  not  they  require  different  concentrations  of  the  con- 
stituents in  the  nutrient  solution  for  their  several  best  growths 
is  yet  not  clearly  shown.  It  now  seems  probable  that  to  some 
extent  at  least  these  basic  mineral  nutrients  can  replace  one 
another  for  the  plant's  metabolism.  It  has  been  shown  in  the 
case  of  certain  lower  plant  organisms  that  potassium  can  be 
more  or  less  successfully  replaced  by  rubidium  and  caesium,  and 
in  the  case  of  some  higher  plants,  possibly  calcium,  magnesium 
and  potassium  can  partially  replace  one  another.1  In  spite  of 
the  fact  that  sodium  as  well  as  potassium  is  a  necessary  con- 
stituent for  the  metabolism  of  higher  animals  which  feed  upon 

1  For  a  more  detailed  discussion  of  this  subject,  and  the  functions  of 
the  several  ash  constituents  in  plant  nutrition,   see :    The  physiology  of 
plants,  by  W.  Pfeffer,  translated  by  A.  J.  Ewart,  vol.  i,  p.  410,  et  seq.,  1900. 
6 


74  THE   SOIL   SOLUTION 

plants,  it  is  generally  held  that  sodium  can  not  replace  potassium 
in  the  processes  of  plant  growth,  although  Wheeler  and  his 
colleagues  have  advanced  evidence  to  show  that  a  partial  replace- 
ment is  possible.1  It  seems  evident,  however,  that  no  gen- 
eralizations can  hold  concerning  the  effect  of  the  concentration 
of  any  one  base  on  plant  growth  which  do  not  include  recog- 
nition of  possible  modifications  due  to  the  presence  of  other 
bases;  and  the  formulation  of  such  generalizations  must  needs 
wait  upon  a  more  thorough  knowledge  of  the  parts  played  by 
the  several  mineral  nutrients  in  the  metabolism  of  different 
classes  of  plants. 

As  to  forms  or  chemical  combinations  in  which  the  inorganic 
constituents  of  the  soil  solution  are  best  adapted  to  plant  growth, 
but  little  can  yet  be  said  other  than  that  the  different  combi- 
nations do  have  an  importance.  Some  empirical  information  is 
available,  such  as  for  instance,  that  potassium  sulphate  or  car- 
bonate is  a  better  fertilizer  for  some  crops  than  is  potassium 
chloride.  It  is  known  that  the  mineral  nutrients  in  the  plant  are 
partly  in  inorganic  combinations  but  largely  in  organic  combi- 
nations. But  the  causal  relationships  are  yet  to  be  worked  out. 
And  finally,  although  some  meagre  experimental  data  have  been 
obtained  as  to  the  effect  of  certain  inorganic  constituents  on  the 
absorption  of  others,  by  particular  plants,  the  mechanism  of 
absorption  itself,  including  the  selective  powers  of  the  plant,  is 
yet  wanting  an  adequate  explanation. 

*The  effect  of  the  addition  of  sodium  to  deficient  amounts  of  potas- 
sium, upon  the  growth  of  plants  in  both  water  and  sand  culture,  by  B.  L. 
Hartwell,  H.  J.  Wheeler  and  F.  R.  Pember,  Report  Rhode  Island  Agri- 
cultural Experiment  Station,  1906-7,  pp.  299-357. 


Chapter  X. 

THE  BALANCE  BETWEEN  SUPPLY  AND  REMOVAL  OF 
MINERAL  PLANT  NUTRIENTS. 

The  mechanism  of  the  solution  and  transport  of  mineral  nu- 
trients developed  in  the  preceding  pages  makes  it  of  interest  to 
determine  the  relation  between  the  possible  or  probable  supply 
of  mineral  plant  nutrients  and  crop  demands  over  large  areas. 
The  inquiry  can  be  formulated  more  specifically  :  Is  the  move- 
ment of  mineral  plant  nutrients  towards  the  surface  soil  equal  to 
or  in  excess  of  the  removal  by  drainage  waters  and  garnered 
crops?  Satisfactory  data  are  yet  wanting  for  anything  like 
exact  computations,  but  approximate  figures  are  available  which 
appear  sufficient  for  the  present  purpose. 

The  rainfall  (R)  can  be  considered  as  disposed  in  three  por- 
tions, the  fly-off  (/),  the  run-off  (r),  and  the  cut-off  (c).  Stat- 
ing this  as  an  equation, 


The  cut-off  can  be  resolved  into  the  portion  (a)  seeping  through 
the  soil  to  ultimately  join  the  run-off,  and  the  portion  (&)  re- 
turning to  the  surface  to  ultimately  join  the  fly-off.  Stated  as 
equations, 

R=f+r+a+b 


In  other  words,  the  rainfall  can  also  be'  considered  as  made  up 
of  the  fly-off,  the  capillary  water  of  the  soil  and  the  drainage 
from  the  area.  According  to  Murray,1  Geikie,2  Newell,3  and 
others,  the  drainage  water  for  humid  areas,  or  such  an  area  as 
the  United  States  as  a  whole,  would  be  between  20  and  30  per 
cent,  of  the  rainfall,  the  major  portion  coming  from  seepage 
water  rather  than  surface  drainage.  Assuming  the  higher  fig- 

1  On  the  total  annual  rainfall  on  the  land  of  the  globe,  and  the  rela- 
tion of  rainfall  to  the  annual  discharge  of  rivers,  by  Sir  John  Murray, 
Scot.  Geog.  Mag.,  3,  65-77  (1887). 

2  Textbook  of  Geology,  by  Sir  Archibald  Geikie,  p.  484  (1903). 

3  In  Principles  and  conditions  of  the  movements  of  ground  water,  by 
F.  H.  King,  Ann.  rept.  U.  S.  Geol.  Surv.,  19,  II,  59-294  (1897-98). 


76  THE  SOU,  SOLUTION 

ure,  and  making  the  further  very  probable  assumption  that  the 
capillary  water  in  the  soil  (&)  is  never  less  than  the  fly-off  or 
the  water  that  evaporates  during  rain  (/),  it  follows  from  the 
equations  given  that  the  capillary  water  is  at  least  35  per  cent, 
of  the  rainfall.  If  we  assume  the  lower  value  for  the  drainage, 
then  the  capillary  water  is  at'  least  40  per  cent,  of  the  rainfall, 
and  if  we  assume  the  extreme  case — that  the  fly-off  is  practically 
negligible — the  capillary  water  becomes  80  per  cent,  of  the  rain- 
fall. It  appears,  therefore,  that  in  all  probability  the  proportion 
of  the  cut-off  water  which  returns  to  the  surface  as  film  water 
or  capillary  water  is  always  greater,  and  generally  much  greater, 
than  the  portion  which  seeps  through  the  soil  to  join  the  run- 
off. 

From  the  available  data,  it  appears  that  the  average  concern- 
tration  of  the  run-off  waters  of  the  United  States  is  about  1.8 
parts  per  million  of  potassium  (K)  and  about  0.6  part  per  mil- 
lion of  phosphoric  acid  (PC^),1  while  the  concentration  of  the 
capillary  groundwater  is  some  ten  or  twelve  times  greater.  But 
even  if  these  concentrations  were  the  same,  it  is  altogether  prob- 
able that  very  much  the  greater  part  of  the  mineral  plant  nu- 
trients dissolved  by  meteoric  waters  is  continually,  if  slowly, 
moving  towards  the  surface  of  the  soil. 

The  average  rainfall  of  the  United  States  may  be  taken  as  ap- 
proximately 30  inches.2  If  it  be  assumed  that  the  discharge 
into  the  sea  is  25  per  qent.,  then  the  capillary  cut-off  water  is 
at  least  37.5,  and  probably  nearer  70  per  cent,  of  the  rainfall. 
King's  experimental  work3  indicates  that  the  higher  figure  is 
much  nearer  the  truth.  Computing  from  the  concentrations 
just  cited,  with  the  equations  given  above,  it  is  found  that  ap- 
proximately 3,500,000  tons  of  potassium  (K)  and  1,200,000  tons 

1  Estimated  from  data  in  Bull.  No.  330,  U.  S.  Geological  Survey,  The 
data  of  geochemistry,  by  Frank  Wigglesworth  Clarke,  1908,  pp.  53-90. 

*  The  latest  authoritative  statement  is  that  the  average  annual  rain- 
fall of  the  United  States  is  29.4  inches ;    see :    Water  Resources,  by  W.  J. 
McGee,  vol.  i,  pp.  39-49,  and  Distribution  of  rainfall,  by  Henry  Gannett, 
vol.  2,  pp.  10-12,  Report  of  the  National  conservation  commission,  Senate 
doc.  No.  676,  6oth  Congress,  2d  session,  1909. 

*  King :    loc.  cit,  p.  85. 


MINERAL  PLANT  NUTRIENTS  77 

of  phosphoric  acid  (PO4)  are  carried  into  the  sea  annually  from 
the  United  States,  while  from  48,00x3,000  to  100,000,000  tons 
of  potassium  and  18,000,000  to  40,000,000  tons  of  phosphoric 
acid  are  being  carried  towards  the  surface  of  the  soil.  If  it  be 
assumed  that  an  average  of  one  ton  per  acre  of  dry  crop  con- 
taining one  per -cent,  potash  and  0.6  per  cent,  phosphoric  acid1 
be  removed  from  the  entire  area  of  the  United  States,  then  the 
annual  loss  from  this  source  would  be  24,000,000  tons  of  po- 
tassium and  14,000,000  tons  of  phosphoric  acid.  Consequently, 
there  is  an  ample  margin  between  the  losses  by  cropping  and 
seepage  waters,  and  the  supply  of  capillary  waters.  It  is  true 
that  cases  exist  where  the  production  of  vegetable  matter  is  much 
greater  than  a  ton  to  the  acre,  productions  of  five  tons  or  even 
more  being  on  record.  But  such  cases  occur  only  where  the 
water  supply  is  also  greater,  either  through  natural  rainfall  or 
artificial  irrigation;  and  it  should  also  be  borne  in  mind  that  the 
production  of  so  large  a  mass  of  green  crop  involves  a  consid- 
erable drawing  power  on  the  water  in  the  soil  in  addition  to  the 
evaporation  which  would  take  place  at  the  surface  under  ordi- 
nary conditions.  In  other  words,  the  plant  would  then  be  play- 
ing no  small  part  in  drawing  to  itself  its  needed  supplies  of  water 
and  dissolved  mineral  nutrients. 

The  question  may  be  asked,  if  the  processes  outlined  above 
are  generally  operative,  why  accumulations  of  soluble  mineral 
substances  are  not  usually  found  at  the  surface  of  the  soil.  As 
a  matter  of  fact  such  accumulations  do  occur  normally  when  the 
evaporation  at  the  surface  is  relatively  large,  that  is,  under  arid 
conditions.  And  under  humid  conditions  it  appears  to  be  a  gen- 
eral rule  that  the  surface  soil  contains  more  readily  soluble  or 
absorbed  mineral  matter  than  do  sub-soils.2  No  great  accumula- 

1  Estimated  from  Wolff's  tables,  How  crops  grow,  by  Samuel  W. 
Johnson,  1890,  appendix. 

1  See,  for  instance :  Investigations  in  soil  management,  by  F.  H.  King, 
Madison,  Wis.,  1904,  p.  62  et  seq.  This  tendency  towards  a  higher  content 
of  absorbed  soluble  mineral  matter  in  the  surface  soil  has  been  amply 
confirmed  by  other  experiments.  It  has  been  advanced  as  an  argument 
against  the  assumption  that  the  hydrolysis  of  the  soil  minerals  is  a 
reversible  process.  But  as  pointed  out  elsewhere  in  the  text,  many  of 
the  soil  minerals  can  be  made  in  the  wet  way  at  more  or  less  elevated 
temperatures  and  the  more  rational  explanation  is  simply  that  at  ordinary 
temperatures  the  rate  of  formation  is  exceedingly  slow. 


78  THE   SOIL   SOLUTION 

tion  occurs  at  the  surface  normally  under  humid  conditions  be- 
cause the  rainfall  is  sufficiently  distributed  throughout  the  year 
to  enable  the  cut-off  water  to  carry  back  promptly  into  the  lower 
soil  levels  any  excessive  amount  of  soluble  material,  there  to 
start  anew  its  slower  ascent  towards  the  surface. 

Calculations  such  as  those  here  presented  are  at  the  best  open 
to  many  objections,  and  it  is  wise  to  avoid  giving  them  too  much 
emphasis.  So  far  as  the  available  data  justify  any  conclusion, 
however,  it  appears  that  the  rise  of  capillary  water  is  entirely 
capable  of  maintaining  a  sufficient  supply  of  mineral  nutrients  for 
crop  requirements;  and  furthermore,  it  is  obvious  that  the  prob- 
lem of  the  supply  of  mineral  plant  nutrients  is  dynamic  and  can- 
not be  successfully  attacked  by  considerations  which  are  essen- 
tially static. 


Chapter  XL 

THE  ORGANIC  CONSTITUENTS  OF  THE  SOU  SOLUTION. 

The  organic  substances  in  the  soil  are  tissue  remains,  to  a  large 
extent  of  plants,  and  to  a  less  extent  of  animals;  and  it  is  to  be 
expected  that  there  may  be  found  also  in  the  soil  the  substances 
which  were  in  the  organisms  at  the  time  of  their  death,  and  de- 
gradation and  decomposition  products  derived  from  these.  More- 
over, there  are  to  be  anticipated  numerous  products  of  bacterial 
origin,  secretions  of  algae,  fungi,  etc.,  so  that  the  organic  com- 
plex in  the  soil  may  contain  numerous  substances  of  widely 
different  chemical  characteristics.  Degradation  products  of  pro- 
teins, fats,  and  carbohydrates,  as  well  as  decomposition  products 
may  be  expected  in  almost  any  soil.  But  it  does  not  follow  that 
any  particular  organic  substance  (excluding,  of  course,  carbon 
dioxide  or  nitrates)  is  to  be  found  in  every  soil.  No  general- 
ization regarding  the  organic  substances  in  the  soil  can  be  made 
such  as  that  formulated  for  the  inorganic  compounds.  It  is 
probable  that  further  investigation  will  show  qertain  organic 
substances  or  classes  of  substances  to  be  common  to  most  soils, 
but  it  is  reasonably  certain  that  many  other  organic  substances 
will  be  found  in  only  a  few  soils,  or  occasionally,  and  these  latter 
will  be  often  a  prominent  factor  characterizing  the  particular 
soil  in  which  they  may  occur. 

Although  no  broad  generalization  is  justified  regarding  the 
composition  of  the  soil  solution  with  respect  to  organic  sub- 
stances dissolved,  nevertheless  the  extension  of  the  methods 
developed  in  the  study  of  the  inorganic  substances  dissolved  has 
led  to  a  considerable  knowledge  of  the  organic  ones. 

In  view  of  the  facts  shown  in  the  preceding  chapters,  and 
at  the  same  time  recognizing  that  good  and  poor  soils  respec- 
tively must  show  differences  in  the  soil  solution  if  the  funda- 
mental thesis  is  valid  as  to  the  relation  of  soils  to  crop  produc- 
tion, experiments  have  been  made  to  investigate  in  a  comparative 
way  solutions  obtained  from  good  and  poor  soils  of  the  same 
type,  locality,  and  physical  characteristics.  For  this  purpose 
two  samples  of  soil  were  taken  from  adjacent  fields  which  had 


8O  THE  SOIL  SOLUTION 

been  under  observation  for  two  years.  The  soils  were  of  the  same 
type,  Cecil  clay,  and  were  so  similar  in  their  physical  charac- 
teristics as  to  be  distinguished  with  difficulty  in  the  laboratory. 
On  one  field  a  good  crop  of  wheat  was  grown,  followed  by  a 
good  crop  of  clover  and  tame  grasses.  On  the  other  field,  the 
corresponding  crops  had  been  quite  poor.  The  field  yielding 
the  good  crops  had  been  plowed  somewhat  deeper,  and  had 
previously  received  a  moderate  application  of  stable  manure. 
Otherwise,  so  far  as  could  be  learned,  the  cultural  history  of  the 
fields  had  been  the  same.  For  convenience,  the  sample  from 
the  first  field  will  be  designated  "good,"  and  from  the  other 
"poor." 

Aqueous  extracts  from  these  soils  were  prepared,  the  same 
proportion  of  distilled  water  to  soil  being  taken  in  each  case, 
and  the  time  of  contact  being  the  same.  The  solutions  were 
freed  from  suspended  matter  by  being  passed  through  Pasteur- 
Chamberland  bougies  under  pressure.  Young  wheat  seedlings 
germinated  at  the  same  time,  and  selected  carefully  for  uni- 
formity of  size  and  apparent  vigor,  were  grown  in  these  solu- 
tions for  three  days.  At  the  expiration  of  this  period  the  seed- 
lings in  the  extract  from  the  good  soil  were  about  five  inches  in 
height,  and  the  roots  were  clear,  clean  and  turgid.  The  plants 
in  the  poor  extract  were  scarcely  three  inches  in  height,  and  the 
roots  were  assuming  a  slimy,  unhealthy  appearance  and  becom- 
ing flaccid  at  the  tips.  The  plants  were  then  all  removed,  the 
roots  washed  carefully  in  tap  water;  the  plants  which  had  been 
in  the  poor  solution  were  placed  in  the  good  solution,  and  those 
which  had  been  in  the  good  solution  were  placed  in  the  poor  solu- 
tion. At  the  end  of  four  days  further,  the  poor  plants  had  sur- 
passed in  height  the  ones  which  had  previously  been  in  the  good 
solution,  and  the  roots  had  acquired  the  general  characteristics  of 
healthy  plants.  These  which  had  been  originally  in  the  good 
solution  and  then  transferred  to  the  poor,  had  made  little  addi- 
tional growth,  and  the  roots  had  become  somewhat  flaccid.1 

This  experiment  was  repeated  several  times,  not  only  with 

*The  success  of  this  and  of  many  of  the  following  experiments  was 
due  in  large  measure  to  the  skill  and  patience  of  Mr.  James  E.  Breazeale. 


ORGANIC   CONSTITUENTS  OF   THE  SOIL   SOLUTION  8l 

the  soils  cited  but  with  samples  from  adjacent  good  and  poor 
spots  in  fields  on  several  soil  types  from  widely  separated  areas ; 
for  instance,  Cecil  clay  from  near  Statesville,  North  Carolina; 
Sassafras  loam  from  Maryland;  Windsor  sand  from  Delaware; 
and  similar  results  were  obtained.  In  other  words,  these  water 
cultures  produced  plants  which  showed  much  the  same  differ- 
ences, in  kind  and  degree,  as  had  been  observed  in  the  field. 
This  was  recognized  as  an  important  step  forward,  for  it  indi- 
cated that  whatever  was  making  a  difference  in  the  crop-produc- 
ing power  of  these  soils  in  the  field  was  transmitted  to  their 
aqueous  extracts,  and  methods  for  studying  the  chemical  prop- 
erties of  solutions  are  far  in  advance  of  methods  for  studying 
mixtures  of  solids. 

The  soil  extracts  described  above  were  subjected  to  a  careful 
analysis  for  their  mineral  constituents.  They  were  found  to  be 
practically  identical  in  this  respect.  Further,  the  poor  extract 
contained  decidedly  more  nitrates  than  the  good — from  three 
to  four  times  as  much.  It  follows,  therefore,  that  the  difference 
in  the  soils  which  produced  a  good  and  a  poor  crop  respectively, 
was  not  due  to  a  difference  in  mineral  plant  nutrients,  or  other 
mineral  differences  probably,  nor  to  their  respective  content  of 
nitrates.  Consequently,  the  poor  solution  was  such,  not  because 
of  the  lack  of  anything,  but  because  of  the  presence  of  some- 
thing inimical  or  "toxic"  to  plant  growth;  and  further,  this 
something  must  be  an  organic  substance  or  substances  more  or 
less  soluble  in  water.  This  conclusion  was  confirmed  in  the 
following  way. 

Samples  of  the  poor  solution  from  the  soil  obtained  near 
Statesville,  N.  C.,  were  diluted  twice,  five  times,  and  ten  times, 
and  wheat  seedlings  were  grown  in  these  solutions,  using  a  sam- 
ple of  the  good  solution  as  a  check.  It  was  found  after  several 
days  growth  that  the  plants  in  the  solution  diluted  tenfold  were 
about  as  good,  or  perhaps  slightly  better,  than  those  grown  in 
the  check  solution.  In  every  case  diluting  the  poor  solution 
had  improved  it  for  plant  growth,  and  the  higher  the  dilution 
the  greater  the  improvement,  in  spite  of  the  consequent  dilution 
of  the  mineral  plant  nutrients.  The  only  explanation  of  these 


82  THE  SOIL   SOLUTION 

results  which  has  yet  suggested  itself  is  that  the  toxic  organic 
substances  present  were  less  effective  on  dilution  until  the  con- 
centration reached  a  point  where  they  actually  became  stimula- 
tive, as  is  common  with  toxins  of  every  character. 

Another  set  of  experiments  confirmed  the  conclusion  that  the 
poor  solution  contained  some  organic  substance  inhibitory  to 
plant  growth.  A  number  of  water  cultures  was  prepared  from 
the  aqueous  extract  of  the  poor  soil,  and  lime  in  various  forms 
was  added  to  the  cultures.  To  two  of  the  cultures  lime  car- 
bonate and  lime  sulphate  respectively  were  added  in  excess,  so 
that  there  was  in  each  case  a  powdered  solid  at  the  bottom  of 
the  containing  vessel.  At  the  end  of  two  days  the  wheat  seed- 
lings which  were  growing  in  the  vessels  containing  the  pow- 
dered solids  had  decidedly  outstripped  those  growing  in  all  the 
others,  the  tops  having  the  appearance  of  unusually  good  and 
healthy  plants.  The  roots  were  of  a  very  remarkable  character, 
being  exceptionally  long,  very  turgid,  clear,  clean  and  translu- 
cent. 

At  once,  new  experiments  were  carried  out  in  which  there 
were  added  to  the  poor  solution,  precipitated  ferric  hydroxide 
freed  from  all  adhering  salts,  precipitated  alumina,  shredded 
filter-paper,  absorbent  cotton,  or  carbon  black.  In  every  case  the 
same  result  was  obtained  as  before,  a  much  improved  growth 
of  top  and  a  vastly  better  root  development.  Since,  by  no 
possibility  could  these  various  added  substances  have  increased 
the  concentration  with  respect  to  mineral  nutrients,  another  ex- 
planation must  be  sought.  Aside  from  their  insolubility,  the 
one  properly  common  to  these  various  substances  was  the  large 
amount  of  surface  they  brought  into  contact  with  the  solution. 
The  one  obvious  explanation  of  their  effects  on  the  growth  of 
the  wheat  seedling,  therefore,  is  that  they  withdrew  or  ab- 
sorbed from  the  solution  some  substance  or  substances  deleteri- 
ous to  plant  growth.  As  diluting  with  respect  to  mineral  nutri- 
ents could  not  possibly  be  expected  to  improve  the  cultural  value 
of  the  solution,  the  conclusion  seems  evident  that  the  effect  pro- 
duced by  these  various  absorbents  was  due  to  more  or  less  com- 
plete removal  from  the  solution  of  organic  substances  inhibitory 


ORGANIC   CONSTITUENTS   OF  THE  SOIL  SOLUTION  83 

to  plant  growth.  These  experiments  were  then  repeated  in 
a  modified  form  by  shaking  the  poor  solution  with  such  ab- 
sorbents as  precipitated  ferric  oxide  or  carbon  black  and  filtering 
before  adding  the  seedling  plants.  The  solutions  thus  pre- 
pared proved  very  satisfactory  nutrient  media,  although  the  de- 
cided elongation  of  the  roots,  always  observed  when  the  ab- 
sorbents were  in  contact  with  the  solutions,  was  not  so  notice- 
able with  these  filtered  solutions. 

.The  experiments  just  described  were  repeated  with  extracts 
from  a  number  of  soils  which  were  supporting  or  had  recently 
supported  poor  crops.  The  accumulated  mass  of  evidence  ad- 
mits of  no  doubt  that  in  many  cases  the  apparent  lack  of  fertility 
of  a  soil  is  due  to  the  presence  of  some  organic  substance  or 
substances  soluble  in  soil  water.  This  point  established,  there 
was  studied  the  effect  of  fertilizers  when  added  to  aqueous  ex- 
tracts from  poor  soils. 

A  large  amount  of  experimenting  has  been  done  on  this  sub- 
ject. It  has  been  found  that  the  common  commercial  fertilizers, 
as  well  as  many  other  substances,  when  added  to  the  soil  ex- 
tract containing  growing  plants,  sometimes  improve  the  plants, 
sometimes  the  contrary.  But,  in  general,  those  particular  sub- 
stances which  improve  any  given  soil  for  a  crop  also  improve  the 
aqueous  extract  of  the  soil  for  the  growth  of  the  same  crop  plant ; 
*.  e.,  should  a  soil  be  known  to  respond  well  to  the  application  of 
superphosphates  when  planted  to  wheat,  then  the  probability  is 
great  that  the  aqueous  extract  of  the  soil  will  be  improved  as  a 
culture  medium  for  the  wheat  plant  by  addition  of  calcium 
phosphate.  Particularly  important  in  this  connection  are  cer- 
tain experiments  with  organic  fertilizers. 

A  soil  which  had  been  found  to  be  quite  unproductive  with 
regard  to  wheat  and  ordinary  tame  grasses  yielded,  however, 
a  much  better  growth  of  plants  if  pyrogallol  or  better  pyrogallol 
and  lime  were  added  to  the  soil  some  days  before  planting.  An 
aqueous  extract  of  this  soil  tested  with  young  wheat  seedlings 
produced  but  a  poor  growth,  as  did  the  soil  itself.  But  with 
the  addition  of  pyrogallol  or  pyrogallol  and  lime  to  the  soil  ex- 


84  THE   SOIL   SOLUTION 

tract,  and  especially  if  the  extract  so  treated  were  allowed  to 
stand  for  a  few  days  with  free  access  of  air,  there  was  obtained 
a  culture  medium  which  yielded  remarkably  good  results  with 
wheat  seedlings.  Not  only  was  there  an  excellent  and  increased 
development  of  tops,  but  the  roots  of  the  seedlings  grown  in  the 
solution  treated  with  pyrogallol  were  unusually  long,  turgid, 
clear  and  translucent.  Here,  then,  there  was  obtained  an  in- 
creased amount  and  improved  character  of  growth  by  the  addi- 
tion of  a  substance  which  contained  only  carbon,  hydrogen  and 
oxygen,  and  no  recognized  plant  food.  Other  organic  substances, 
such  for  instance  as  tannin,  gave  similar  results. 

With  the  recognition  that  the  presence  of  organic  dissolved 
substances  in  the  nutrient  medium  produced  effects  on  a  grow- 
ing plant  of  as  great  or  even  greater  magnitude  than  those  pro- 
duced by  inorganic  dissolved  substances,  there  was  carried  out  a 
number  of  experiments  to  test  more  specifically  such  substances 
as  might  reasonably  be  expected  to  be  present  naturally  in  soils. 
The  results  thus  obtained  suggested  experiments  with  other  re- 
lated substances.  The  first  substance  to  suggest  itself  is  stable 
manure.  Taking  it  all  in  all,  this  substance  is  probably  the  most 
efficient  as  well  as  the  most  generally  used  soil  amendment  in 
the  experience  of  mankind.  The  good  effects  produced  by  this 
substance  have  in  the  past  been  generally  considered  as  due  to 
the  readily  "available"  potash,  phosphoric  acid  and  nitrogen  it 
contains,  but  thoughtful  experimenters  and  agriculturists  have 
long  doubted  that  this  explanation  is  sufficient,  since,  after  all 
the  mineral  constituents  of  stable  manure  are  usually  small  in 
amount,  and  out  of  all  proportion  to  the  effects  resulting  from 
its  use.  That  some  of  the  results  are  due  to  an  improvement  in 
the  physical  condition  of  the  soil  when  manure  is  used  has  quite 
rightly  been  generally  assumed ;  but  to  its  content  of  nitrogenous 
components  its  value  has  in  the  main  been  ascribed. 

A  well-fermented  aqueous  extract  of  stable  manure  was  pre- 
pared, and  filtered  free  of  suspended  solids.  Four  equal  vol- 
umes of  this  solution  were  taken.  Three  of  these  portions  were 
evaporated  to  dryness  in  platinum  dishes,  and  the  residues 


ORGANIC   CONSTITUENTS   OF  THE  SOIL   SOLUTION  85 

incinerated.  To  the  dishes  containing  the  ash  were  added  re- 
spectively nitric  acid,  sulphuric  acid,  and  hydrochloric  acid  in 
slight  excess,  and  the  dishes  again  brought  to  dryness.  Water 
cultures  for  wheat  seedlings  were  then  prepared.1  Into  one  was 
introduced  the  given  volume  of  manure  extract;  into  another  the 
ash  from  an  equal  volume  of  the  extract  which  had  subsequently 
been  treated  with  nitric  acid;  and  cultures  with  the  ash  which 
had  been  treated  respectively  with  sulphuric  and  hydrochloric 
acid  were  similarly  prepared.  After  ten  days  growth,  the  plants 
from  the  several  cultures  were  compared.  The  plants  from  the 
cultures  which  contained  the  sulphates  and  the  chlorides  were 
not  materially  different  from  the  plants  grown  in  the  check 
culture.  The  plants  from  the  nitrate  culture  had  larger  shoots, 
but  shorter  roots  than  the  check  plants.  But  the  plants  grown 
in  the  culture  to  which  the  manure  extract  had  been  added  direct- 
ly had  by  far  larger  and  better  shoots  and  the  roots  were  incom- 
parably superior  to  those  grown  in  any  other  culture,  being 
larger,  thicker,  better  branched,  clear,  bright  and  translucent,  and 
very  turgid,  "very  like  the  roots  obtained  in  cultures  to  which  car- 
bon black  or  precipitated  ferric  oxide  had  been  added. 

The  results  of  this  experiment,  which  has  been  repeated  a 
number  of  times,  using  manure  extracts  of  various  origins,  leave 
no  doubt  that  it  is  the  organic  components  of  the  manure  which 
produce  the  characteristic  effects,  for  the  ash  culture  contained 
all  and  even  more  of  the  mineral  constituents  "available"  in  the 
original  extract,  and  the  nitrate  culture  excluded  any  explana- 
tion based  on  the  nitrogenous  content  of  the  manure.  This  con- 
clusion was  supported  by  the  results  of  another  experiment. 

To  a  manure  extract  was  added  alcohol,  which  precipitated 
most  of  the  organic  dissolved  substances  but  very  little  of  the 
inorganic  ones.  The  precipitated  organic  matter  was  filtered 
off,  dried  carefully  in  a  water  oven  to  eliminate  the  alcohol,  and 
then  taken  up  in  sufficient  water  to  equal  the  original  volume 
of  manure  extract.  The  filtrate  containing  the  major  part  of  the 

1  Further  studies  on  the  properties  of  unproductive  soils,  B.  E.  Liv- 
ingston et  al.,  Bull.  36,  1907,  and  48,  1908,  Bureau  of  Soils,  U.  S.  Dept. 
Agriculture. 


86  THE  SOIL  SOLUTION 

salts  was  boiled  vigorously  to  eliminate  the  alcohol  and  water 
was  then  added  to  restore  the  original  concentration.  A  third 
solution  was  prepared  by  bringing  together  the  organic  and  in- 
organic substances  which  had  previously  been  separated  as  above 
described/  The  three  solutions  were  used  as  water  cultures  for 
wheat  seedlings,  a  solution  of  the  original  manure  extract  being 
taken  for  a  check  culture.  The  original  manure  extract  and  the 
reconstructed  manure  extract  gave  plants  of  about  equal  de- 
velopment. The  culture  containing  the  organic  dissolved  sub- 
stances only,  gave  plants  of  nearly,  but  not  quite,  equal  de- 
velopment to  those  grown  in  the  check  culture.  But  the  plants 
grown  in  the  solution  containing  the  dissolved  minerals  only, 
while  fine  plants  and  making  what  would  ordinarily  be  considered 
a  good  development,  were  decidedly  smaller  as  regards  their 
aerial  parts,  and  the  roots  were  in  no  wise  comparable  to  the 
roots  of  the  plants  grown  in  the  cultures  containing  the  dissolved 
organic  substances. 

This  last  experiment  has  been  repeated,  with  dissolved  sub- 
stances prepared  from  another  manure  extract,  but  in  this  case 
the  organic  and  inorganic  substances  were  separated  by  dialysis. 
This  suggested  yet  another  experiment,  in  which  it  was  sought 
to  hasten  the  process  of  dialysis,  by  introducing  electrodes  into 
the  manure  extract,  each  electrode  being  surrounded  by  some 
porous  membrane,  either  of  parchment  paper,  or  unglazed  por- 
celain. Not  only  were  the  mineral  constituents  of  the  manure 
extract  readily  separated  in  this  way,  passing  into  the  electrode 
chambers,  as  did  also  to  some  slight  extent  organic  compounds, 
but  also  about  the  outer  walls  of  the  electrode  chambers  there 
was  marked  segregation  and  deposition  of  organic  materials. 
The  organic  substances  deposited  at  the  cathode  were  found  to 
stimulate  greatly  the  growth  of  wheat  seedlings  while  those  de- 
posited at  the  anode  were  found  to  retard  the  growth  of  seed- 
lings. It  seems  probable,  therefore,  that  stable  manure  con- 
tains organic  components  which  produce  as  great  or  greater 
effects  upon  growing  plants  as  do  the  inorganic  substances  it 
contains :  that  on  the  whole  these  organic  components  induce 


ORGANIC   CONSTITUENTS   OF   THE  SOIL   SOLUTION  87 

increased  plant  growth,  but  some  of  them,  by  themselves  alone, 
would  retard  plant  growth. 

In  a  similar  way  green  manures  have  been  examined.  If 
fresh  clover,  alfalfa,  or  cowpeas,  be  macerated  and  an  aqueous 
extract  thus  prepared,  it  will  in  general  be  quite  toxic  to  plants 
such  as  wheat;  and  if  this  extract  be  allowed  to  stand  and  fer- 
ment or  sour  the  resulting  solution  will  be  totally  unfit  for  the 
growth  of  seedling  plants.  But  if  the  clover,  alfalfa,  or  cow- 
pea  vines  be  allowed  to  wilt  thoroughly  before  being  macerated 
and  extracted,  or  if  they  be  macerated  and  incorporated  with 
soil  and  allowed  to  remain  thus  for  ten  days  or  a  fortnight  be- 
fore being  extracted;  then,  the  resulting  solution  will  be  quite 
stimulating  to  such  plants  as  wheat,  corn  or  the  grasses,  when 
added  either  to  water  or  soil  cultures.  It  would  seem,  therefore, 
that  the  mineral  constituents  of  the  legumes  commonly  employed 
as  green  manures  are  less  important  than  the  organic,  in  affect- 
ing the  growth  of  crops  subsequently  planted,  and  the  inhibitory 
or  toxic  action  of  fresh  green  manure  seems  to  be  recognized  in 
the  common  practice  of  waiting  some  days  after  turning  under 
a  green  manure  crop  before  seeding  to  a  new  crop. 

The  wilting  of  a  green  manure  involves  a  darkening  and  some 
blackening  of  the  mass,  with  apparently  some  absorption  of 
oxygen.  This  fact  has  suggested  a  trial  of  other  organic  sub- 
stances which  show  a  decided  ability  to  absorb  oxygen.  Among 
such  substances,  pyrogallol  stands  preeminent.  It  has  been 
shown  that  when  pyrogallol,  or  better  pyrogallol  and  lime,  is 
added  to  certain  soils,  naturally  low  in  productive  power,  and 
allowed  to  stand  for  a  few  days,  these  soils  are  readily  brought 
into  good  condition  and  support  good  crops  of  wheat,  rye,  or 
grasses.  Pyrogallol  in  water  cultures'  is  rather  toxic  to  wheat 
plants,  even  in  quite  dilute  solutions.  But  if  the  aqueous  solu- 
tion of  pyrogallol  be  allowed  to  stand  exposed  to  the  air,  and 
better  if  the  solution  be  made  slightly  alkaline  as  by  the  addi- 
tion of  lime,  oxygen  is  absorbed,  and  a  dark  brown  or  blackened 
solution  is  soon  formed,  which  is  stimulating  to  wheat  seedlings. 
Many  experiments  have  indicated  it  to  be  a  general  rule  that 


88  THE  SOII,  SOLUTION 

soluble  organic  substances  which  are  toxic  to  plant  growth  yield 
oxidation  products  which  are  harmless  or  positively  beneficial. 

The  suggestion  has  been  made  that  the  well-known  infertility 
of  subsoils,  when  freshly  turned  up,  is  caused  by  the  presence 
of  alkaloids  of  the  purine  or  codeine  type,  due  to  the  activities 
of  anaerobic  bacteria.  Water  cultures  and  pot  cultures  show 
that  while  these  substances  do  have  a  marked  effect  on  plant 
growth,  it  is,  frequently,  quite  beneficial ;  strychnine  for  example, 
in  certain  concentrations,  produces  a  very  decided  stimulation  in 
the  growth  of  wheat  seedlings.  It  is  clear  that  some  other  ex- 
planation will  have  to  be  sought  for  the  lack  of  fertility  of  sub- 
soils. 

A  number  of  the  substances  which  may  be  expected  for  one 
reason  or  another  to  be  present  in  soils,  have  been  investigated  as 
to  their  effect  on  plants.  In  this  connection  may  be  cited  the 
work  of  Livingston1  and  of  Dachnowski,2  who  have  studied  the 
effect  on  vegetation  of  the  organic  substances  dissolved  in  bog 
waters.  In  the  following  table  are  given  the  results  obtained 
by  growing  wheat  seedlings  in  solutions  containing  some  one  of 
a  number  of  substances  which  might  be  expected  to  occur  in  a 
soil  or  to  be  derivatives  of  such  substances.  It  will  be  observed 
that  in  the  case  of  these  dissolved  organic  substances,  as  has 
been  repeatedly  established  with  the  inorganic  ones,  in  concentra- 
tions sufficiently  dilute  not  to  be  toxic,  they  generally  show 
the  opposite  effect  and  appear  to  be  stimulating. 

1  Physiological  Properties  of  Bog  Water,  by  B.  E.  Livingston,  Bot. 
gaz.,  39,  348-355  (iQOS). 

2  The  toxic  property  of  bog  water  and  bog  soil,  by  Alfred  Dachnow- 
ski, Bot.  gaz.,  46,  130-143  (1908). 


ORGANIC    CONSTITUENTS 


THE   SOU,   SOLUTION 


89 


4 

a, 


. 

O    PL, 

«    O 


W   o 

fr  s 

85  ,° 

p  H 
P* 

P     BJ 

M 

cn   a 


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** 


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>  W 


O 

s 

fa 

fa 

w 


C3 


§§ 

8    !£jj    *2 

0 

V 

q 

Remarks 

ormal  growth  in  c 
centration  below 

p.  u.  111. 

o  injury  below  I, 
p.  p.  m. 
Dps  of  all  pla 
good.  Roots  sligh 
injured  at  hlg 
concentrations 
nly  roots  were  in  ju 

at  500  p.  p.  m. 
o  injurious  action 

S 

1 

V    M 

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THE  SOIL  SOLUTION 


tJ                                  ° 

S-g                  s 

en 

£^               •* 

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Prt  i7                                                  3    tfi 

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S                            .Q     --2 

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O                             w  CX<U                                 §  *- 

i 

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IsPil 

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a       

p.                          '.                                                  1 

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P.                                                      "                         M*                      '                         ' 

s 

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S. 

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s    JTa 

*Q                                                                                 M                      HH                      M 

Q      « 

w 

o 

/\      K 

w"                     w   ^  A 

Compound 

v          g^       o    5-cJW    J5j,r    B«    W 

C              **        \          ./   ^V.     ^^                                  ...  ,  /^           2j        y^\. 
T™           H^             X     ^^          ^L    f  j                  ^^  ^^fj  ^T           *~f*      /f     \ 

\/  Ijtj    9  <~>    c»a       d          c;    W°    a 
^   .a  \x  M    >r'fc    ^ja          ..os 

iL      "3        Z          S       "     .  X>      »rt                               ri 

J  1  3  Z\/^    S  g  g  v 
ail    8  *    8/    a"    5 

-                                «r         cu             ""         ^ 
S      a      «           S      S                        S 

•C     'C     .2          S     '3        6            §        2 

Sped               o^             J3                  3             ctf 

^        aj             JI^        ^             rt                 <h            *i4 

»     K     CQ         <  •    O        x                      to 

««J                •*             'Oi     »«t           **                 8            8 

ORGANIC  CONSTITUENTS  OF  THE  SOII,  SOLUTION 


Remarks 

cuJS  g                                                               o,     a  g 

O~             CJ  »^ 
O                  M     <^J 

10  o)   a>                                                                                                  10        O 
O   u 
fe  S  S                                                                                                    w         0   0 
O-«  '                                                                                        >           A 

^.ja                                                              o      a  Ma 
aa^                                                         -S    "2^ 

.2   *   °                                                                                                           0)        "«   * 

3gM                                                                                                                                                                  ^5_J^*OO< 

M                                                       >2    li*' 

Concentration 
causing 
greatest 
stimulation 

a    :           o              :           «          :           :      : 

0.       ; 

IvOwest 
concentration 
causing 
injury 

s 

O                   O                       «O                 10               w>                 O        O 
a,     in               O                    w                «                               -<t      o     . 

IO                                                                                                                                                                   M 

a 

I,owest 
concentration 
causing 
death 

a                          0                       °                  2               2 
a      •              5                  >o             o            o               • 

p.    :           °             «          «         «o          ;      : 

.            •                              M                                                                                                                                      •               • 

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S*.  £  a 

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^             O                   v                 v 

•*                                                                                                                      ,-«                           T- 

<u               aT                 .5                .3 
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H3                ^3                      h                                  a                  n       .5 

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y                     OH                  O<           3                          S 
0                                                                                                      S\       O 

THE  SOIL  SOLUTION 


T3 
V 

2 

08 

a 

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5 

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P. 

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P. 

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0 

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10  O 

cH   O 

M 

^  C  S'^> 

P. 

M 

M 

HfO  g  — 

a 

8 

a 

o 

M  33  M 

22.25 

a'    888      8          8 

8    8 

8  : 

8 

K  C  S  W 

P,      kO  IO  IO           IO                    M 

IO           M 

M*W  S"° 

p. 

8 

a      i 

O       u*' 

•£*.  ti  a 

^      <S   0   ro          C\                  t^ 

O          CT> 

00    N 

00 

E  °  &S 

Cd          *H    »-*    M 

^ 

M 

3       S  8 

•o 

Q 

cn  : 

o  : 

o  : 

x-x         10 

«M                    *• 

B 

a  : 

O 

Compound 

Pyrocatechin,  C6H4(OH)2(r,: 
Arbutin,  CuH16OT  
Phloroglucin,  C6Hs(OH)5(i,3 

xCHO 

Vanillin,  C6H,—  O.CHS  
\)H 

xCOOH 

Vanillic  acid,-  C8H8—  O.CHS 

o 
o 

?ffl 

Oo—C 

»  Xi/ 

-     (J 
'S      47 

a    § 

o     .2 

'3        3 

a   01 

•Cinnamic  acid,  C6H5CH  :  CH 
Sodium  cinnamate  

O          \ 
\ 

°\         / 

\/ 

a" 
1 

i 

a 

V     <0  -VI                   3 

R>      S 

^ 

»^ 

ORGANIC   CONSTITUENTS   OF  THE   SOU,   SOLUTION 


93 


(ii 

a| 

in 

,9 

|1| 

V 

«    '.2* 

tf 

!i*3 

*o   .-5 
«  ex  ? 

c 

§        g 

'•s  M»-j3 

a        .        .              :     :  :  : 

11  §1 

P. 

e  S  SfrS 

P. 

o         » 
O 

lowest 
concentration 
causing 
injury 

a                   O                    M                                     M           M    IO  O 

>O                                                                      1-1 

p. 
p. 

c 

>  "^1  w  C3 
5  H  3  D 

3       :       8             8    888 

P.              •               10                         >->        >->    CO  10 

^  a  ^*° 

P. 

8 

Mi 

q      v 

>,             <N              CO                         t^       O  00  00 

O 

1 

ffi    0            | 
0    •      c.       -1- 

a 

o 

0 

w    &T  1           ^ 

0     2,    o1           w 

c?     sfa  §     A  w"  so 

^r-'O   O   O  ,?°  <u 

.3       °  -3    \  1  /       n"  S 
|        B"|     ^    -sJ^ 

f      ||      °"       §|| 

°        iS                "^^ 

94  THE  SOIL   SOLUTION 

a.  Aspartic  acid  has  been  found  in  young  sugar-cane  and  in  seedlings 
of  the  bean  and  pumpkin. 

b.  Asparagine  was  first  found  in  asparagus ;  but  has  since  been  shown 
to  be  relatively  abundant  in  many  species. 

c.  Glycocoll  is   one  of  the   simpler  and   more  common   degradation 
products  of  proteins. 

d.  Alanine  is  a  common  degradation  product  of  proteins  and  is  related 
chemically  to  phenylalanine,  and  to  tyrosine,  which  has  been  found  in 
many  plants. 

e.  Leucine,  an  amino-acid  of  a  paraffine  series  and  a  decomposition 
product  of  proteids,  has  been  found  in  certain  mushrooms,  vetches,  lupine, 
gourds,  potatoes,  corn,  etc. 

/.  Tyrosine  is  an  important  decomposition  product  of  proteids,  is 
widely  distributed  and  found  in  many  plants  and  fungi. 

g.  Choline  is  a  derivative  of  certain  lecithins  and  is  found  in  many 
seeds  and  growing  plants. 

h.  Neurine  is  a  substance  closely  related  to  choline,  and  probably 
formed  from  it. 

i.  Betaine  is  closely  related  to  both  choline  and  neurine,  and  is  found 
in  many  seeds  and  plants. 

j.  Alloxan  is  closely  related  chemically  to  convicine,  which  latter  is 
found  in  beets  and  certain  beans. 

k.  Guanine  is  a  widely  distributed  nitrogenous  body,  and  has  been 
found  in  the  seeds  of  vetch,  alfalfa,  clover,  gourds,  barley,  sugar-beets 
and  sugar-cane. 

/.  Xanthine,  a  substance  closely  related  to  guanine,  has  been  found  in 
a  number  of  plants. 

m.  Guanidine,    a   substance   chemically   related   to   guanine,   has   been 
found  in  a  number  of  plants  of  different  species. 

n.  Skatol  is  a  derivative  of  proteids  and  is  a  common  product  of  the 
activities  of  some  varieties  of  bacteria. 

o.  Pyridine  has  been  shown  to  exist  in  soils,  as  such  probably,  by 
Shorey,  who  obtained  it  from  certain  soils  in  Hawaii. 

p.  Ricin  is  found  in  the  castor-oil  plant. 

q.  Mucin  has  been  found  in  yams. 

r.  Pyrocatechin  has  been  found  in  the  bark  of  various  trees,  the 
berries  of  the  Virginia  creeper,  the  sap  of  sugar  beets  and  in  several 
varieties  of  willows. 

s.  Arbutin  has  been  found  in  many  plants,  especially  in  some  of  the 
grasses. 

t.  Phloroglucin  is  easily  derived  from  a  number  of  plant  constituents. 

u.  Vanillin  forms  readily  from  a  glucoside,  which  is  very  widely  dis- 
tributed in  many  plants,  and  by  some  authorities  is  supposed  to  be  a 
product  of  the  decomposition  of  wood  tissues. 


ORGANIC   CONSTITUENTS   Of  THE  SOIL   SOLUTION  95 

v.  Quinic  acid,  which  is  found  with  quinine  in  the  cinchona  bark, 
also  occurs  in  beet  leaves,  certain  hays,  cranberry  leaves,  and  occasionally 
in  other  plants. 

w.  Quinone  has  been  shown  to  result  from  the  action  of  a  certain 
fungus,  Streptothrix  chromogena,  common  in  soils. 

x.  Cinnamic  acid  is  found  in  certain  barks,  and  forms  esters  which 
have  been  found  in  the  leaves  of  various  plants. 

y.  Cumarin  has  been  found  in  a  large  number  of  plants,  including 
the  grasses,  beets,  sweet  clover,  etc. 

z.  Daphnetin  occurs  in  some  species  of  Daphne  and  is  closely  related 
to  cumarin. 

aa.  Esculin,  as  well  as  the  corresponding  esculetin,  has  been  found 
occasionally  in  a  number  of  plants. 

bb.  Heliotropine,  or  piperonal,  has  the  odor  of  heliotrope  and  is  found 
in  flowers. 

cc.  Borneol  occurs  in  needles  of  different  varieties  of  pine,  fir,  spruce 
and  hemlock,  golden  rod  and  thyme. 

dd.  Camphor  is  closely  related  chemically  to  borneol  and  is  secreted 
by  a  number  of  plants;  it  is  found  in  the  wood  of  Cinnamomum,  cinna- 
mon root,  in  the  leaves  of  sassafras,  spikenard,  rosemary,  rosewood,  etc. 

ee.  Turpentine  is  a  constituent  of  many  plants  and  coniferous  trees. 

Finally,  a  number  of  organic  substances  has  been  isolated  from 
soils.  Their  composition,  and  in  several  cases  their  constitutions 
have  been  determined.  The  effects  of  these  on  plants,  when 
they  are  present  in  the  cultural  media  have  been  studied.  Thus, 
Shorey1  was  able  to  isolate  picoline  carboxylic  acid  (C7H7NO2) 
from  certain  soils  in  Hawaii,  and  this  same  substance  has  since 
been  found  in  several  soils  of  the  United  States.  In  aqueous 
solutions  it  is  quite  toxic  to  wheat  seedlings.  Since  then  a  num- 
ber of  other  definite  organic  compounds  have  been  isolated  from 
soils  belonging  to  at  least  eight  different  classes  of  organic  sub- 
stances, including:2 

Hentriacontane,  C31H64. 

Monohydroxystearic  acid,  CH3(CH2)^CHOH(CH2)9COOH. 

Dihydroxystearic  acid,  CH3(CH2)7CHOH.CHOH.(CH2)7 
COOH. 

1  Organic  nitrogen   in   Hawaiian   soils,   by   E.    C.    Shorey,   report   of 
Hawaii  Experiment  Station,  1906,  37-59. 

2  Chemical  Nature  of  Soil  Organic  Matter,  by  Oswald  Schreiner  and 
Edmund  C.  Shorey,  Bull.  74,  Bureau  of  Soils,  U.  S.  Department  of  Agri- 
culture, 1910. 


g6  THE  SOIL   SOLUTION 

r 

Agroceric  acid,  C21H42O3. 
Paraffinic  acid,  C24H48O2. 
Lignoceric  acid,  C24H48O2. 
Phytosterol,  C26H44O.H2O. 
Pentosan,  C5H8O4. 
Agrosterol,  C26H44O.H2O. 
Picoline  carboxylic  acid,  C7H7O2N. 
Histidine,  C6H9O2N3. 
Arginine,  C0H14O2N4. 
Cytosine,  C4H5ON3.H2O. 
Xanthine,  C5H4O2N4. 
Hypoxanthine,  C5H4ON4. 
Glycerides,  resin  acids,  etc. 

Some  of  these,  picoline  carboxylic  acid,  dihydroxystearic  acid 
and  the  pentosan  just  cited,  are  toxic  to  growing  plants;  others 
are  not.  The  origin  and  mode  of  production  of  these  substances 
in  the  soil  is,  generally  speaking,  uncertain  and  obscure,  and  is 
yet  one  of  the  important  fundamental  problems  confronting  the 
soil  chemist. 

It  is  important  to  note  that  the  organic  substances  thus  far 
isolated  from  soils  are  of  widely  varying  types,  and  with  very 
different  chemical  characteristics.  As  pointed  out  above,  almost 
any  type  of  organic  substance  is  likely  to  be  found  in  soils,  and 
the  effects  of  any  of  them  on  growing  plants  can  hardly  be  pre- 
dicted from  a  priori  considerations. 

It  has  been  found  that  as  a  general  rule  the  continued  growth 
of  one  crop  in  any  soil  results  in  a  low  crop  production.  Pot 
cultures  have  given  even  more  pronounced  results  in  the  same 
direction.  The  explanation  long  accepted  is  that  the  soil  has, 
as  a  result  of  continued  cropping,  become  deficient  in  some  one 
or  more  of  the  "available"  mineral  nutrients.  Pot  experiments, 
where  the  garnered  crop  was  returned  to  the  soil  and  still  a 
diminished  yield  was  obtained,  throw  doubt  on  this  explanation. 
Still  further  doubt  results  from  water-cultures  which,  by  grow- 
ing a  crop  in  them,  become  "poor"  for  subsequent  crops,  although 


ORGANIC   CONSTITUENTS   OF   THE)   SOU,   SOLUTION  97 

there  is  maintained  in  them  an  ample  supply  of  mineral  plant 
nutrients,  and  they  are  easily  renovated  by  good  absorbers. 
These  facts  find  a  more  satisfactory  explanation  as  being  due 
to  the  production  in  the  nutrient  medium  of  deleterious  organic 
substances  originating  in  the  growing  plant  itself.  This  idea 
seems  to  have  been  advanced  first  by  De  Candolle,  in  1832,!  to 
account  for  the  beneficial  results  obtained  by  employing  a 
rotation  of  crops.  It  appears  to  have  been  held  by  Liebig  at 
one  time,  although  he  subsequently  abandoned  it  in  favor  of 
the  view  that  the  benefits  of  a  crop  rotation  are  due  to  the  several 
crops  requiring  different  proportions  of  mineral  nutrients,  and 
that  the  disturbance  of  the  balance  in  the  soil  produced  by  one 
crop  is  not  unfavorable  to  the  growth  of  some  other  crop. 
Although  lacking  direct  experimental  confirmation,  this  latter 
view  of  Liebig's  has  long  prevailed  among  agricultural  inves- 
tigators, partly  by  reason  of  his  authority,  partly  by  reason  of  the 
dominance  of  the  plant-food  theory  of  fertilizers,  and  partly 
by  reason  of  the  fact  that  the  ideas  of  De  Candolle  as  originally 
advanced  included  certain  errors  soon  detected.  The  trend  of 
recent  investigations  has  been  distinctly  in  favor  of  a  modified 
form  of  the  view  of  De  Candolle.  It  has  been  recognized  that 
other  factors  enter  into  crop  rotations,  such  as  the  elimination 
of  associated  weeds,  various  kinds  of  animal,  insect  and  plant 
parasites,  preparation  of  the  soil  by  a  deep-rooted  crop  for  a 
shallow-rooted  following  crop,  etc.  It  has  come  to  be  recog- 
nized that  there  are  natural  associations  of  plants,  and  natural 
rotations  of  vegetation  certainly  determined  by  other  than  plant- 
food  factors.  Thus,  in  the  eastern  United  States,  wheat  is 
followed  by  ragweed  naturally,  while  across  the  fence  cocklebur 
and  wild  sunflower  come  in  after  the  corn,  the  difference  in 
vegetation  being  as  sharply  marked  after  the  removal  of  the 
crops  as  when  they  still  occupied  the  land.  Analyses  of  the 
ragweed,  for  instance,  although  it  is  a  shallower  rooted  crop 
than  wheat,  show  that  it  takes  from  the  soil  as  much  of  the 

1  See  in  this  connection,  Further  studies  on  ,the  properties  of  unpro- 
ductive soils,  by  B.  E.  Livingston,  Bull.  No.  36,  Bureau  of  Soils,  Dept.  of 
Agric.,  1907,  pp.  7-9. 


THE   SOIL,   SOLUTION 


mineral  nutrients  as  does  the  preceding1  wheat  crop.  The  invest- 
igation of  Lawes  and  Gilbert2  on  fairy  rings  showed  that  the 
continual  widening  of  the  rings  can  not  be  satisfactorily  explained 
by  the  comparison  of  the  mineral  constituents  in  the  soil  within 
and  without  the  rings.  Work  at  Woburn3  on  the  effect  of 
grass  on  apple  trees  finds  no  other  plausible  explanation  than 
that  the  growing  grass  produces  in  the  soil  organic  substances 
detrimental  to  young  apple  trees.  A  number  of  similar  cases 
have  been  recorded. 

1  Mr.  J.  G.  Smith  has  made  a  comparison  between  the  potash  and 
phosphoric  acid  content  of  the  wheat  and  following  crop  of  ragweed 
grown  on  a  farm  in  Fairfax  Co.,  Va.  His  unpublished  results,  with  some 
others  found  in  the  literature,  are  given  in  the  following  table : 


Material 

Potash 
K2O 
per  cent. 

Phosphoric 
acid,  P2O6 
per  cent. 

Analyst 

Wheat  

0.76 
I.73 
1.28 

1.18 
1.796 

1.79 
1.809 

0.52 
o-73 
0-35 

0-39 
0.51 

0.41 
0-54 

Smith 
Smith 
Smith 

Smith 
Wolff's  tables  in  Johnson's 
"How  Crops  Grow,"p.376. 
DeRoode,  in  Bull.  19,  W.  Va. 
Agr.  Exp.  Sta.,  1891. 
Burney,  2d.     Ann.  rept.  S. 
C.  Stat.,  1889.  p.  146 

Ragweed  in  seed  and  accoin- 

Winter  wheat  in  flower  .... 

On  the  whole,  ragweed  seems  to  require  and  take  from  the  soil  about 
as  much  mineral  matter  as  does  wheat.  It  is  stated  by  some  of  the 
dairy  farmers  near  Washington,  who  cut  the  mixture  of  ragweed,  other 
weeds  and  grass  following  wheat,  for  a  hay  crop,  that  the  weight  of  the 
ragweed  crop  is  generally  heavier  than  that  of  the  wheat  crop.  There- 
fore the  ragweed  actually  removes  more  mineral  matter  from  the  field 
than  does  the  wheat.  These  facts  lend  no  support  to  the  popular  notion 
that  wheat  "exhausts"  the  soil  of  its  "available"  mineral  plant  nutrients. 
For  analyses  of  a  number  of  common  American  weeds,  see  Analyses  of 
the  ashes  of  certain  weeds,  by  Francis  P.  Dunnington :  Ann.  Chem.  Jour., 
2,  24-27  (1880). 

2  Note  on  the  occurrence  of  "fairy  rings,"  by  J.  H.  Gilbert :  Jour. 
Linn.  Soc.,  15,  17-24  (1875). 

*  Second,  third  and  fifth  reports  of  the  Woburn  Experimental  Fruit 
Farm,  1900,  1903,  1905. 


ORGANIC   CONSTITUENTS   OF  THE  SOIL  SOLUTION  99 

Finally,  although  less  work  has  been  done  in  this  direction 
with  higher  plants  than  with  other  organisms,  it  is  now  recog- 
nized as  a  general  law  of  all  living  organisms  that  they  function 
less  readily  as  the  products  of  their  activities  accumulate.1 
These  products  may,  however,  be  inimical,  neutral  or  even  stimu- 
lating to  other  organisms. 

This  problem  has  been  investigated  critically  by  direct  experi- 

1  It  may  not  be  amiss  to  point  out  here  that  this  general  law  holds 
for  all  dynamic  phenomena.  In  chemistry,  for  instance,  the  general  law 
is  well  recognized  that  the  rate  of  reaction  diminishes  with  increase  in 
the  active  mass  of  the  reaction  products.  It  can  be  shown  that  the  prin- 
ciple applies  to  plant  growth.  Young  plants  will  withdraw  potassium 
more  rapidly  than  chlorine  from  solutions  of  potassium  chloride;  that  is, 
the  solution  soon  contains  free  hydrochloric  acid.  Conversely  the  plants 
cause  a  solution  of  sodium  nitrate  to  become  alkaline.  Therefore,  if  the 
above  principle  holds,  then  the  initial  addition  of  small  amounts  of 
hydrochloric  acid  to  a  solution  of  potassium  chloride  should  slow  up  the 
absorption  of  potassium  by  seedling  wheat  plants,  or  the  addition  of 
sodium  hydroxide  the  absorption  of  nitrogen  from  a  solution  of  sodium 
nitrate.  Mr.  J.  J.  Skinner  has  tested  this  idea  with  the  following  results, 
growing  carefully  selected  wheat  seedlings,  for  3  days  in  solutions  of 
pure  potassium  chloride,  solutions  of  potassium  chloride  containing  initially 
enough  excess  of  hydrochloric  acid  to  be  of  an  N/5,ooo  concentration 
with  respect  to  the  acid,  solutions  of  sodium  nitrate,  and  solutions  of 
sodium  nitrate  containing  initially  an  excess  of  sodium  hydroxide. 

Solutions  of  KC1  containing  80  p.  p.  m.  K»O. 
I     K2O  absorbed  40.0  p.  p.  m. 
2,    K2O  absorbed  40.0  p.  p.  m. 

3  K,O  absorbed  36.3  p.  p.  m. 

Solutions  of  KC1  (80  p.  p.  m.  KaO)  and  HC1  (N/5,ooo). 

4  K2O  absorbed  26.7  p.  p.  m. 

5  KaO  absorbed  29.5  p.  p.  m. 

6  KaO  absorbed  26.7  p.  p.  m. 

Solutions  of  NaNO3  containing  80  p.  p.  m.  NH8. 

7  NH3  absorbed  30.2  p.  p.  m. 

8  NH8  absorbed  30.2  p.  p.  m. 

9  NH3  absorbed  32.5  p.  p.  m. 

Solutions  of  NaNO,  (80  p.  p.  m.  NH3)  and  NaOH  (N/5,ooo). 

10  NH3  absorbed  27.8  p.  p.  m. 

11  NH3  absorbed  34.3  p.  p.  m. 

12  NH3  absorbed  27.8  p.  p.  m. 


100  THE  SOIL  SOLUTION 

mentation,  growing  wheat,  and  other  seedlings  in  water  and  agar 
cultures.1  It  has  been  shown  that  wheat  renders  the  culture 
media  unsuitable  for  subsequent  wheat  crops,  though  it  can  be 
reclaimed  or  renovated  by  treatment  with  such  absorbents  as 
carbon  black,  or  by  other  methods.2  Wheat  did  about  as  well 
when  grown  in  a  medium  which  had  previously  supported  a 
growth  of  cowpeas  as  when  planted  in  a  fresh  medium;  poorer 
results  were  obtained  after  oats;  no  crop  produced  such  poor 
results  in  the  succeeding  wheat  crop  as  did  wheat  itself. 

It  is  yet  a  matter  of  dispute  as  to  whether  the  substances  thus 
added  to  nutrient  media  are  truly  excretory  products  of  the 
plant,  sloughed  off  or  otherwise  eliminated  from  the  surface  of 
the  roots,  or  further  elaborated  by  bacterial  or  other  agencies 
before  becoming  effective.  These  are  important  problems  for 
the  plant  physiologist  and  the  soil  chemist  alike.  It  is  beyond 
dispute,  however,  by  reason  of  a  large  and  increasing  weight  of 
evidence,  much  of  it  .direct  experiment,  that,  as  a  result  of  the 
growing  of  plants,  soils  and  the  soil  water  do  contain  organic 
substances;  harmful  to  the  plant  or  organism  eliminating  them; 
harmful,  innocuous,  or  even  stimulating  to  other  plants  or 
organisms. 

For  the  elimination  from  the  soil  of  toxic  or  inhibitory  organic 
substances,  whether  excreted  by  roots  or  otherwise  produced, 
several  methods  are  more  or  less  effective.  When,  as  is  some- 
times the  case,  the  substance  is  volatile,  it  may  be  removed  by 
heating,  distilling  with  steam,  or  passing  a  current  of  air  through 
the  soil  or  cultural  medium.  These  methods,  while  effective  in 
the  laboratory  and  possibly  applicable  to  green-house  conditions, 
are  naturally  inapplicable  to  field  conditions.  In  this  last  case 
the  obvious  procedure  is  to  increase  as  much  as  possible  the 
absorptive  powers  of  the  soil;  to  secure  the  best  possible  drain- 
age ;  and  with  these,  the  best  possible  aeration  of  the  soil. 

1  Some  factors  in  soil  fertility,  by  Oswald  Schreiner  and  Howard  S. 
Reed,  Bull.  No.  40,  Bureau  of  Soils,  U.  S.  Dept.  Agriculture,  1907. 

*  Soil  fatigue  caused  by  organic  compounds,  by  Oswald  Schreiner  and 
M.  X.  Sullivan:  Jour.  Biol.  Chem.,  6,  39-50  (1909). 


ORGANIC   CONSTITUENTS   OF  THE  SOIL   SOLUTION  IOI 

It  has  been  found  that,  in  general,  a  cultural  medium  which 
has  been  rendered  unfit  for  the  continued  growth  of  a  crop,  is 
readily  renovated  by  treatment  with  oxidizing  agents,  and  is 
sometimes  rendered  even  better  than  ever  by  such  treatment, 
which  would  suggest  that  the  oxidation  products  from  plant 
effluvia  may  be  even  beneficial  to  the  plant.  To  this  end  the 
growing  plant  seems  itself  to  be  an  active  agent,  apparently 
attempting  automatically  to  protect  itself  against  the  products 
of  its  own  activities.  It  has  been  pointed  out  by  Molisch1  that 
root  secretions  have  an  oxidizing  power,  apparently  of  an  enzy- 
motic  character.  Some  doubt  of  the  validity  of  Molisch's  work 
has  been  raised  by  Czapeck,  Pieffer,  and  others;  nevertheless  it 
is  now  accepted  that  while  intercellular  autoxidation  or  reduc- 
tion processes  may  take  place  in  living  roots,  the  higher  plants, 
such  as  our  common  crop  plants,  also  show  a  more  or  less  well- 
developed  extracellular  oxidizing  power  in  the  neighborhood 
of  the  root  tips  and  root  hairs.2  That  this  oxidizing  power  dis- 
played by  growing  roots  is  enzymotic  is  indicated  by  the  fact 
that  artificial  culture  media  frequently  display  it  also  after  plants 
have  been  grown  in  them  for  a  short  while.3 

It  has  been  shown  that  the  oxidizing  action  of  growing  roots 
is  generally  promoted  by  having  the  cultural  medium  slightly 

1t)ber  Wurzelausscheidungen  und  deren  Einwirkung  auf  organische 
Substanzen,  von  Hans  Molisch.  Sitzungsber.  Akad.  Wiss.  Wien,  Math.  nat. 
Kl.,  96,  84-109  (1888). 

'The  role  of  oxidation  in  soil  fertility,  by  Oswald  Schreiner  and 
Howard  S.  Reed :  Bull.  No.  56,  Bureau  of  Soils,  U.  S.  Dept.  Agriculture, 
1909. 

3  From  considerations  as  yet  highly  speculative,  a  different  type  of 
oxidation  by  roots  might  be  anticipated.  It  is  recognized  that  in  the 
absorption  of  mineral  nutrients  by  plants  a  certain  amount  of  selection 
enters.  For  example,  a  plant  with  its  roots  in  a  solution  of  potassium 
chloride,  absorbs  more  potassium  than  chlorine,  relatively,  and  free  hydro- 
chloric acid  is  left  in  the  solution.  Obviously  in  the  absorption,  work 
is  done,  and  a  possible  explanation  is  that  water  is  decomposed  at  the 
absorbing  surface  of  the  root,  with  the  liberation  of  oxygen.  Theoret- 
ically, it  ought  not  to  be  difficult  to  investigate  this  by  a  study  of  the 
energy  changes  during  absorption,  but  growing  plants  do  not  lend  them- 
selves readily  to  such  experimentation. 


102  THE  SOIL  SOLUTION 

alkaline  or  neutral  rather  than  acid.  It  is  also  promoted  by  the 
addition  of  various  minerals  salts,  notably  by  nitrates,  phosphates, 
or  lime  salts.  Potassium  salts  promote  the  oxidation  but  slightly, 
and  in  some  experiments  have  even  produced  a  slight  decrease. 
The  corresponding  sodium  and  ammonium  salts  are  more  favor- 
able than  those  of  potassium.1  It  appears  altogether  probable, 
therefore,  that  the  mineral  salts  in  commercial  fertilizers  may 
have  some  importance  in  this  connection. 

Whatever  may  be  the  role  of  mineral  fertilizers  towards 
organic  substances  toxic  to  growing  plants,  it  is  certain  that  they 
have  an  importance  and  one  that  is  probably  specific,  as  indi- 
cated by  some  recent  investigations.2  Culture  solutions  contain- 
ing the  constituents  potassium,  nitric  acid  and  phosphoric  acid 
were  prepared  in  such  manner  that  they  covered  the  range  of  all 
possible  ratios  of  these  constituents  in  intervals  of  ten  per  cent, 
in  each.  Into  one  set  of  these  solutions  was  introduced 
dihydroxystearic  acid,  into  another  set  cumarin,  and  into  a  third 
set,  vanillin,  and  into  a  fourth  set,  quinone.  The  growth  of 
wheat  seedlings  in  these  several  sets  showed  indubitably  that 
these  several  organic  substances  which  are  all  deterrent  to  the 
growth  of  wheat,  were  modified  in  their  influence  by  the  presence 
of  the  mineral  salts ;  but  that  nitrates  were  more  efficient  than  the 
other  minerals  in  the  case  of  the  solutions  containing  dihydroxy- 
stearic acid  or  vanillin;  phosphates  were  most  efficient  in  the 
case  of  the  solutions  containing  cumarin,  and  potassium  most 
efficient  in  solutions  containing  quinone.  As  the  organic  sub- 
stances used  in  these  experiments,  either  in  themselves  or  as 
typifying  classes  of  compounds,  may  be  anticipated  in  soils  under 
natural  conditions,  it  is  again  apparent  that  mineral  fertilizers 
have  a  function  in  addition  to  the  traditional  one  of  increasing 
the  supply  of  mineral  nutrients. 

The  fact  that  the  oxidizing  power  of  roots  is  more  marked 
when  grown  in  aqueous  extracts  of  soils  in  good  tilth  than  in 
extracts  made  from  soils  in  poor  tilth,  shows  that  cultural 

1  Action  of  fertilizing  salts  on  plant  enzymes,  by  M.  X.  Sullivan,  Jour, 
biol.  chem.,  6  (1909),  proceed.  XLIV. 

"Private   communication  by   Dr.   Oswald   Schreiner  and   Mr.   J.   J. 
Skinner. 


ORGANIC   CONSTITUENTS   OF   THE   SOIL   SOLUTION  IO3 

methods  are  no  less  important  in  field  practice  than  are  fertilizers 
in  promoting  this  important  activity  of  plants.  There  is  little 
reason  to  doubt  that  oxidizing  agencies  other  than  plant  roots 
(bacterial  for  instance)  are  more  or  less  active  in  every  arable 
soil,  and  numerous  investigations,  among  which  Russell's 
researches1  are  conspicuous,  leave  little  doubt  that  oxidatoin 
processes  are  promoted  by  good  tilth.  It  is  apparent,  therefore, 
that  by  the  activities  of  the  plant  itself  as  well  as  other  agencies, 
the  general  tendency  in  soils  is  the  destruction  of  or  rendering 
innocuous  harmful  plant  effluvia  or  other  organic  substances,  and 
to  this  end  are  effective  each  of  the  three  methods  of  soil  control 
generally  practiced,  namely,  tillage,  crop  rotation  and  fertilizers. 

Among  the  organic  components  of  the  soil  none  have  greater 
importance  and  interest  than  those  containing  nitrogen  or  as 
they  are  frequently  called  the  nitrogen  carriers.  Conspicuous 
among  these  are  the  nitrates.  While  it  is  now  generally  conceded 
that  ammonia  and  other  nitrogen  compounds  can  be  taken  up 
by  higher  plants  and  elaborated  by  them  under  special  conditions, 
it  nevertheless  remains  true  that  plants  draw  their  needed  supplies 
of  nitrogen  from  the  soil  solution,  mainly  in  the  form  of  nitrates. 
The  problems  presented  by  these  nitrogen  carriers  are  mainly 
bacterial2  and  physiological,  but  certain  features  are  of  direct 
importance  to  the  soil  chemist  and  to  a  study  of  the  soil  solution. 
It  is  now  known  generally  that  there  are  many  kinds  of  nitrifying 
and  denitrifying  bacteria  in  soils,  and  that  probably  every  arable 
soil  contains  several  species,  or  varieties  at  least  of  both  kinds. 
With  good  tilth  and  consequent  aerobic  conditions,  nitrifying 
processes  prevail,  and  with  poor  tilth  or  in  subsoils,  anaerobic 

1  Oxidation  in  soils,  and  its  connection  with  fertility,  by  Edward  J. 
Russell:  Jour.  Agric.  Sci.,  1,  261-279  (1905)  ;  Pt.  II.  The  influence  of 
partial  sterilization,  by  Francis  V.  Darbishire  and  Edward  J.  Russell,  2, 
305-326  (1907). 

*  The  fixation  of  atmospheric  nitrogen  by  bacteria,  by  J.  G.  Lipman, 
Bull.  81,  Bureau  of  Chemistry,  U.  S.  Dept.  of  Agriculture,  1904,  pp.  146- 
160;  A  review  of  investigations  in  soil  bacteriology,  by  Edward  B.  Voor- 
hees  and  Jacob  G.  Lipman,  Bull.  194,  Office  of  Experiment  Stations,  U.  S. 
Dept.  of  Agriculture,  1907. 


IO4  THE   SOIL   SOLUTION 

conditions  and  denitrifying  processes  prevail.  Warmth,  moisture, 
the  reaction  of  the  soil,  and  perhaps  other  factors  markedly  affect 
the  activity  of  the  organisms  of  the  soil  solution.  Another 
important  factor  is  that  the  absorptive  powers  of  the  higher 
plants  are  markedly  affected  by  sunlight,  so  that,  especially  on 
bright  and  clear  days,  there  is  generally  a  higher  concentration 
of  nitrates  in  the  soil  solution  in  the  morning  than  in  the  evening. 
This  fact  would  seem  to  affect  seriously  the  value  of  some  recent 
and  extensive  investigations  where  it  has  been  sought  to  classify 
soils  by  their  content  of  water-dissolved  nitrates.  Nitric  acid  is 
more  readily  leached  from  soils  than  are  most  other  acid  radicals. 
Consequently  nitrates,  like  other  organic  components  of  the  soil 
solution,  and  unlike  'inorganic  components,  tend  to  vary  greatly 
in  concentration. 


Chapter  XII. 

FERTILIZERS. 

It  is  generally  recognized  that  the  great  practical  problem  con- 
fronting the  soil  chemist  is  the  proper  use  of  soil  amendments  or 
fertilizers.  The  farmers  of  the  United  States  now  spend 
annually  for  fertilizers  upwards  of  $100,000,000.  It  is  estimated 
by  various  authorities  that  a  large  fraction,  perhaps  as  much 
as  three- fourths,  of  the  material  represented  by  this  expenditure 
is  misapplied  for  lack  of  intelligent  direction.  Yet  all  of  this 
enormous  mass  of  fertilizers  can  be  used  to  advantage.  Great 
as  it  is,  it  is  relatively  small  beside  the  total  which  will,  and 
must,  be  used  in  a  not  distant  future,  with  the  growth  and 
development  of  intensive  methods  of  cultivation  consequent  upon 
the  rapid  settling  of  the  country,  the  practical  disappearance  of 
new  lands  and  the  increase  in  money  value  of  the  old  lands. 
The  commercial  importance  of  the  problem,  therefore,  makes  it 
desirable  that  special  emphasis  should  be  given  to  fertilizers  from 
the  point  of  view  developed  in  the  preceding  chapters.  It  should 
be  recalled  that  the  use  of  fertilizers  constitutes  one  of  the 
three  great  general  methods  of  soil  control,  and  further  that 
while  tillage  methods,  crop  rotations,  and  fertilizer  applications 
can  be  used  to  supplement  one  another,  no  one  of  these  methods 
can  be  expected  to  take  satisfactorily  the  place  of  another. 

Crop  production  is  dependent  upon  a  large  number  of  factors. 
Upon  the  rainfall,  both  as  to  the  amount  and  distribution;  upon 
the  sunlight,  as  to  amount  and  distribution;  upon  the  chemical 
and  physical  properties  of  the  soil;  soil  bacteria  and  other 
biologic  agents;  enzymes  in  the  soil;  biological  factors  in  the 
plant,  and  probably  many  other  things.  Opinions  do  and  will 
continue  to  differ  as  to  what  these  factors  are,  but  at  least  every 
one  agrees  that  they  are  many. 

Attempting  to  formulate  these  factors  develops  fundamental 
difficulties,  since  it  is  not  positively  known  how  far  the  variables 
are  dependent  or  independent,  and  we  have  no  idea  as  to  the 
nature  of  the  function  or  functions.  The  weight  of  existing 


IO6  THE  SOIIv   SOLUTION 

evidence  favors  the  view  that  all  the  factors  are  dependent 
variables,  although  numerous  attempts  have  been  made  from  time 
to  time  to  show  that  some  one  factor,  such  as  the  rainfall  for 
instance,  or  the  mean  annual  temperature,  or  available  plant- 
food,  is  practically  an  independent  factor.  Although  it  should  be 
rather  easy  to  determine  experimentally  the  nature  of  the  func- 
tion, if  any  of  these  various  factors  were  independent,  this  has 
never  been  done,  and  this  fact  is  itself  a  strong  argument  that 
all  the  factors  in  crop  production  are  dependent  on  one  another. 

When  there  is  introduced  into  the  equation  a  factor  for  any 
one  of  the  methods  of  soil  control,  i.  e.,  tillage,  crop  rotation, 
or  fertilizers,  it  becomes  even  more  apparent  that  the  function 
is  determined  by  dependent  variables,  for  the  new  factor  always 
more  or  less  affects  several  if  not  all  of  those  already  cited. 
For  instance,  fertilizers  certainly  affect  the  chemical  properties 
of  the  soil,  its  physical  properties,  the  soil  bacteria,  perhaps  the 
plant- food  supply,  the  oxidation  of  plant  effluvia  and  other 
factors.  It  is  obvious,  therefore,  that  a  satisfactory  theory  of 
fertilizer  action  can  not  be  a  simple  one  but  must  of  necessity  be 
complex ;  and  the  same  statement  is  no  less  true  as  regards  tillage 
and  crop  rotation. 

The  recognition  of  the  fact  that  the  action  of  fertilizers  is  a 
complex  function  depending  upon  many  factors  and  groups  of 
factors  which  vary  among  themselves  and  with  each  individual 
soil,  carries  with  it  the  conviction  that  an  exact  or  quantitative 
fertilizer  practice,  while  theoretically  possible,  is  probably 
unattainable  since  methods  for  the  solution  of  such  complex 
functions  are  generally  wanting.  It  is  not  surprising,  therefore, 
that  the  empirical  experience  of  the  past  has  failed  to  develop  a 
quantitative  practice.  However  disappointing  this  may  seem  at 
first  sight,  the  prospect  is  not  altogether  hopeless,  for  this  point 
of  view  indicates  a  systematic  scheme  for  experimentally  deter- 
mining a  qualitative,  but  nevertheless  rational,  fertilizer  practice. 
The  dominance  of  the  plant-food  theory  of  fertilizers  in  the 
past,  shutting  off,  as  it  has,  a  rational  attack  of  the  problem,  is 
causing  the  annual  waste  of  millions  of  dollars  in  misapplied 
fertilizers,  and  it  is  of  scarcely  less  economic  than  scientific 


FERTILIZERS  107 

importance  to  investigate  and  extend  our  knowldege  of  the  effect 
of  soil  amendments  upon  the  many  factors  in  crop  production. 
With  a  knowledge  of  the  effect  of  fertilizers  upon  the  physical, 
chemical  and  biological  factors  in  crop  production,  and  of  the 
nature  of  the  interdependence  of  these  factors,  will  come  the 
ability  to  manage  intelligently  the  individual  field  for  the  part- 
icular crop.  This  knowledge  can  only  come  by  attacking  the 
problem  from  the  dynamic  view-point,  and  so  far  as  the  soil 
factors  are  concerned,  they  can  apparently  be  studied  best  as 
they  affect  the  properties  of  the  soil  solution. 

While  it  seems  certain  that  some  fertilizer  effects  are  directly 
upon  the  soil  and  secondarily  upon  the  plants,  it  cannot  be  doubted 
that  in  others,  the  phenomena  are  more  directly  concerned  with 
the  absorption  by  and  the  metabolism  within  the  plant  and  until 
these  plant  processes  are  better  understood,  nothing  approaching 
a  satisfactory  practice  can  be  anticipated.  Why  and  how  plants 
exercise  the  selective  powers  they  appear  to  possess  are  funda- 
mental questions  yet  to  be  answered.  The  important  effects 
sometimes  produced  by  adding  to  the  nutrient  medium  such 
substances  as  manganese  salts  which  are  not  necessary  to  the 
growth  of  the  plant,  can  no  more  be  neglected  than  the  study  of 
the  phosphorus  needs.  The  presence  in  the  soil  universally  of 
substances  other  than  the  recognized  mineral  nutrients,1  may 
very  well  have  a  significance  for  plant  production  hitherto 
unsuspected,  for  the  fact  that  an  organism  can  continue  to 
function  in  the  absence  of  a  substance  is  no  argument,  much 
less  proof,  that  it  would  not  function  better  with  that  substance 
present.  Recent  investigations,  showing  that  animal  organisms 
are  sometimes  more  resistant  to  certain  toxins  and  diseases  under 
starvation  conditions  or  when  ingesting  substances  unnecessary 
to  normal  development,  suggest  the  possibility  at  least  of  similar 
phenomena  with  plants.  It  is  at  any  rate  clear  that  the  practical 
problem  of  the  best  production  of  plants  from  soils  is  not  merely 
one  of  providing  a  relatively  large  supply  of  potassium,  phos- 
phorus and  nitrogen. 

1  See,  for  instance,  Barium  in  soils,  by  G.  H.  Failyer,  Bull.  No.  71, 
Bureau  of  Soils,  U.  S.  Dept.  of  Agriculture,  1910. 


IO8  THE  SOIL  SOLUTION 

In  this  connection  it  is  well  to  consider  what  constitutes  a 
commercial  fertilizer.  It  must  be  a  substance  the  addition  of 
which  either  directly  or  indirectly  affects  the  properties  of  the 
soil  or  the  growing  plant ;  it  must  be  obtainable  in  large  quantities 
and  from  a  source  or  sources  of  supply  not  readily  exhausted; 
and  it  must  be  cheap.  Of  the  many  substances  filling  the  first 
condition,  all  those  which  fulfill  also  the  other  conditions  are 
used  as  fertilizers,  with  the  exception  of  common  salt  and  human 
excrement.  In  spite  of  the  fact  that  it  does  not  contain  a  con- 
ventional plant-food,  sodium  chloride  appears  to  produce  results 
quite  similar  to  those  produced  by  the  usual  fertilizer  salts.  Its 
use  has  been  followed  generally  by  an  increased  yield  of  crop, 
but  occasionally  by  a  decreased  one,  and  it  appears  not  improb- 
able that  further  investigation  would  show  sodium  chloride  to 
have  a  considerable  value  as  a  fertilizer.  Human  excrement  or 
night  soil,  and  the  sewage  and  garbage  refuse  of  our  large  cities 
are  not  commercial  fertilizers,  although  having  undoubtedly  a 
high  agricultural  value.  Objection  has  been  urged  to  them  that 
they  are  "filthy"  and  liable  to  contain  dangerous  pathogenic 
organisms.  Both  objections  could  be  met.  It  seems  a  more 
rational  explanation  that  the  agricultural  methods  of  this  country 
have  not  yet  become  sufficiently  intensive  to  necessitate  the  con- 
servation of  such  materials  or  to  justify  their  commercial 
exploitation. 

New  products  will  come  into  use  from  time  to  time,  as  in  the 
case  of  calcium  cyanamid  and  basic  calcium  nitrate.  But  it  is 
worthy  of  note  that  these  substances  have  become  available  not 
so  much  because  of  their  agricultural  value,  but  incidentally  to 
the  efforts  of  inventors  and  manufacturers  to  produce  cheap 
nitric  acid  for  the  preparation  of  high  explosives.1  There  seems 

1  In  this  connection  it  may  be  of  interest  to  call  attention  to  the  fact 
that  the  Twelfth  Census  shows  less  than  a  fifth  of  the  sodium  nitrate 
brought  into  the  United  States  goes  into  the  fertilizer  trade.  Moreover, 
the  production  of  ammonium  salts  by  the  extensive  coke  and  gas  plants 
of  the  country  has  been  practically  nil  not  because  of  any  inherent  diffi- 
culties in  making  them  or  because  the  cost  of  production  has  been  high, 
but  because  the  market  demands  in  this  country  have  been  too  small. 


FERTILIZERS  lOO, 

no  reason  to  doubt  that  an  ample  supply  of  desirable  substances 
will  always  be  available  for  fertilizer  purposes.  The  immediate 
practical  problem  for  the  future  is  not  the  seeking  of  new  fer- 
tilizers but  the  rational  use  of  those  at  hand. 


Chapter  XIII. 

ALKALI. 

In  the  preceding  chapters  there  have  been  considered  the 
phenomena  which  obtain  under  humid  conditions.  Under 
exceptional  conditions  of  prolonged  drought  there  occurs  an 
accumulation  of  soluble  mineral  substances  at  or  near  the  sur- 
face of  the  soil.  This  phenomenon  is  pronounced  in  arid  and 
semi-arid  regions,1  and  the  accumulations  of  soluble  salts  oc- 
curring in  such  regions  is  known  in  the  United  States  as 
"alkali,"  in  India  as  "reh,"  in  Africa  as  "brak,"  and  in  other 
countries  by  various  local  designations.  The  study  of  the  ex- 
treme conditions  producing  alkali  has  added  materially  to  the 
present  knowledge  of  the  processes  taking  place  in  soil  of  humid 
areas.  Moreover,  alkali-infested  areas  are  themselves  becoming 
of  so  much  importance  with  the  growing  needs  for  further  new 
lands,  that  it  seems  wise  to  give  here  an  outline  of  the  chemi- 
cal principles  involved  in  their  soil  solutions.2 

Alkali  is  sometimes  a  single  salt,  but  usually  a  mixture  of 
some  two  or  more  of  the  chlorides,  sulphates,  carbonates,  bi- 
carbonates,  and  occasionally  the  nitrates,  phosphates  and  borates, 
of  sodium,  magnesium,  potassium,  and  calcium,  and  occasionally 
strontium  and  lithium.  In  the  United  States,  when  the  carbonate 
of  sodium  is  present  to  an  appreciable  extent,  the  salt  mixture  is 
known  as  black  alkali,  in  contradistinction  to  white  alkali,  which 
latter  does  not  contain  sodium  carbonate.3  Generally,  but  not 

1  Occasional   occurrence    of   alkali   in   humid   regions,   by   Frank   K. 
Cameron,  Bull.  No.  17,  Bureau  of  Soils,  U.  S.  Dept.  Agriculture,   1901, 
PP-  36-38.     This  phenomenon  should  not  be  confused  with  the  surface 
deposition   of   various   kinds   of   saline   material    from   springs,   which   is 
fairly  common  in  both  humid  and  arid  regions,  the  world  over. 

2  Alkali   soils   of   the   United   States,   by   Clarence   W.   Dorsey,   Bull. 
No.  35,  Bureau  of  Soils,  U.  S.  Dept.  Agriculture,  1906. 

8  Black  alkali  is  so  called  because  the  caustic  solution  containing 
sodium  carbonate,  in  rising  to  the  surface  of  the  soil,  dissolves  and 
carries  with  it  organic  matter  which  is  subsequently  left  on  the  surface 
in  more  or  less  blackish  deposits,  often  ring-like  in  appearance.  It  is  by 
no  means  uncommon,  however,  to  find  deposits  of  "black  alkali"  which 
are  not  black  at  all,  and  it  is  quite  common  to  find  "white  alkali"  so  dark 
in  color  as  to  suggest  the  presence  of  sodium  carbonate,  although  the 
latter  be  absent. 


III 

always,  soils  containing  alkali  also  contain  accumulations  of  the 
less  soluble  salts,  calcium  carbonate,  or  calcium  sulphate,  or  a 
mixture  of  the  two.  These  substances,  sometimes  cementing 
the  less  soluble  mineral  components  of  the  soil,  sometimes  almost 
pure,  are  found  in  layers  more  or  less  continuous,  and  from  a 
fraction  of  an  inch  to  several  feet  in  thickness,  in  a  position 
approximately  parallel  to  and  at  a  moderate  depth  below  the 
surface  of  the  soil.  In  such  cases  these  layers  form  a  "hard-pan" 
and  frequently  the  treatment  of  this  type  of  hard-pan  is  the  most 
difficult  and  vexing  problem  in  the  management  of  alkali-bear- 
ing soils. 

The  origin  of  alkali  is  often  uncertain.  In  some  cases  the 
geological  evidences  in  the  area  make  it  certain  that  the  alkali 
came  from  the  desiccation  of  former  bodies  of  sea  water  which 
had  become  isolated  from  the  ocean.  In  other  cases  the  alkali 
appears  to  come  from  the  desiccation  of  lakes  which  are  the  de- 
positories of  the  drainage  of  a  surrounding  area,  and  which  have 
no  outlet  to  the  sea.  In  still  other  cases  it  has  been  supposed 
that  the  alkali  is  derived  from  wind-borne  sea-spray.  Various 
explanations  of  a  more  or  less  special  character  with  regard  to 
particular  localities  or  circumstances  are  to  be  found  in  the  litera- 
ture.1 

The  chemical  principles  involved  in  the  desiccation  of  a  body 
of  sea  water  are  now  pretty  well  understood,  owing  mainly  to 
the  investigations  of  van't  Hoff,  Meyerhoffer,  and  their  co- 
workers.2  The  salts  in  sea  water  and  those  constituting  "white 
alkali"  are  mainly  the  chlorides  and  sulphates  of  sodium,  potas- 

1An  interesting  case  is  the  Billings  Area,  Montana,  where  the  alkali 
seems  to  be  derived  from  the  oxidation,  solution  and  subsequent  hydroly- 
sis of  the  pyrites  and  marcasite  of  the  neighboring  Pierre  shales.  The 
sulphuric  acid  thus  formed,  leaching  through  shales  and  sandstones,  takes 
up  various  bases  and  the  predominating  salts  in  the  alkali  of  this  area 
are  the  sulphates  of  sodium  and  magnesium. 

2  Zur  Bildung  der  ozeanischen  Salzablagerungen,  von  J.  H.  van't  Hoff, 
Braunschweig,  1905-09.  For  a  detailed  discussion  of  these  results  with 
reference  to  alkali  deposits  see :  Calcium  sulphate  in  aqueous  solutions, 
by  Frank  K.  Cameron  and  James  M.  Bell,  Bull.  No.  33,  Bureau  of  Soils, 
U.  S.  Dept.  Agriculture,  1906. 


112  THE  SOIL  SOLUTION 

sium  and  magnesium.  Calcium  is  also  present,  appearing  in  deep 
deposits  as  anhydride,  and  at  the  surface  as  gypsum. 

From  the  results  of  this  work  it  is  possible  to  predict  the 
order  in  which  the  different  salts  or  minerals  will  separate 
from  the  evaporating  solution.  At  ordinary  temperature 
(25°  C)  the  first  salt  to  be  deposited  from  the  dilute  solution 
is  gypsum  (CaSO4.2H2O)  followed  by  halite  or  sodium  chloride 
(NaCl)  in  quantity.  Sodium  chloride  continues  to  separate  at 
all  higher  concentrations.  Next  will  be  deposited  kainite 
(MgSO4KC1.3H2O).  At  the  concentration  then  reached,  the 
stable  sulphate  of  calcium  is  anhydride  (CaSO4),  which  con- 
tinues to  separate  from  solution  as  desiccation  proceeds.  Con- 
sequently, if  the  gypsum  previously  deposited  is  yet  in  con- 
tact with  the  solution,  it  tends  to  be  transformed  to  anhydrite 
and  at  all  higher  concentrations  the  deposition  of  anhydrite  may 
be  expected.  As  evaporation  proceeds  a  point  is  reached  where 
kainite  and  kieserite  (MgSO4.H2O)  separate.  Further  evapora- 
tion brings  a  concentration  at  which  kieserite  and  carnallite 
(MgCl2.KC1.6H2O)  are  precipitated,  and  as  the  process  pro- 
ceeds, finally  the  point  is  reached  where  kieserite,  carnallite  and 
bischofite  (MgCl2.6H2O)  all  three  separate  with  sodium  chloride. 
The  final  products  separating  at  a  higher  temperature,  83°  C., 
are  the  same  four  solids,  sodium  chloride,  kieserite,  carnallite 
and  bischofite.1  The  alternate  layers  of  anhydrite  and  sodium 
chloride  noticeable  in  some  desiccated  sea  beds  is  probably  the 
result  of  alterations  in  temperature,  anhydrite  being  less  soluble, 

*It  will  be  interesting  to  compare  with  the  above  the  following  brief 
description  of  the  Stassfurt  salt  deposits,  taken  from  Ries's  Economic 
Geology  of  the  United  States  (1905),  p.  127.  "At  the  bottom  is  the 
main  bed  of  rock  salt  which  is  broken  up  into  layers  2-5  inches  thick 
by  layers  of  anhydrite.  Above  this  come  200  feet  of  rock  salt,  with 

which  are  mixed  layers  of  magnesium  chloride  and  polyhalite Resting 

on  this  is  180  feet  of  rock  salt,  with  alternating  layers  of  sulphates 
chiefly  kieserite,  the  sulphate  of  magnesia.  These  layers  are  about  I  foot 
thick.  Lastly,  and  uppermost,  is  a  135-foot  bed  consisting  of  a  series 

of  reddish  layers  of  rock  salts  ef  magnesia  and  potassium,  kainite 

kieserite carnallite tachhydrite as  well  as  masses  of  snow-white 

boracite." 


AI,KAU  113 

and  sodium  chloride  somewhat  more  soluble  in  hot  than  in  cold 
water.  During  warm  weather  there  would  be  a  greater  tendency 
for  anhydrite  to  separate  and  in  colder  weather  for  sodium  chlo- 
ride to  be  precipitated.  Anhydrite  at  the  surface  would  gradually 
absorb  water  vapor  from  the  atmosphere  and  be  transformed  to 
gypsum.1 

Besides  the  principal  salts  just  described,  there  may 
separate  at  one  concentration  or  another  other  various  dou- 
ble salts  including  langbeinite  (2MgSO4.K2SO4),  polyhalite 
(K2SO4.MgSO4.2CaSO4.2H2O),  glauberite  (CaSO4.Na2SO4), 
syngenite  (CaSO4.K2SO4.H2O),  potassium  pentasulphate 
(K2SO4.5CaSO4.H2O,  krugite  (4CaSO4.K2SO4.MgSO4.2H2O), 
and  possibly  others.  These  are  all  stable  over  very  restricted 
ranges  of  concentration,  however,  and  if  formed,  probably  sel- 
dom persist,  but  pass  over  to  more  stable  salts  as  the  desicca- 
tion proceeds,  and  have  little  more  than  a  passing  theoretical 
interest. 

The  addition  of  carbonates  to  the  system  introduces  some 
further  modifications.2  In  this  case  lime  carbonate  is  the  first 
salt  to  be  precipitated,  followed  probably  by  the  same  order  of 
deposition  as  outlined  above.  As  the  mother  liquor  becomes 
more  concentrated,  it  apparently  loses  its  alkaline  character,  for 
the  addition  of  an  alcoholic  solution  of  phenolphthalein  does 
not  produce  the  characteristic  red  color.  That  the  solu- 
tion does  actually  contain  dissolved  carbonates  is  shown  by  the 
appearance  of  the  red  color  on  diluting  a  portion  of  the  mother 
liquor  with  distilled  water.  An  interesting  example  in  nature 
is  furnished  by  the  Great  Salt  Lake,  Utah.  A  test  of  the  water 
of  this  lake  in  1899  gave  no  alkaline  reaction  with  phenolphthalein, 

*As  examples,  some  of  the  gypsum  deposits  of  Kansas  may  be  cited, 
according  to  Haworth,  Mineral  resources  of  Kansas,  1897,  p.  61,  and 
the  classical  case  at  Bex,  Switzerland,  described  by  J.  G.  F.  Charpentier, 
Uber  die  Salz-Lagerstatte  von  Bex:  Ann.  Phys.  Chim.,  3,  75-80  (1825), 
and  by  G.  Bischof,  Elements  of  chemical  and  physical  geology,  London, 
1854-58,  Vol.  i,  pp.  350-1. 

*The  action  of  water  and  aqueous  solutions  upon  soil  carbonates, 
by  Frank  K.  Cameron  and  James  M.  Bell,  Bull.  No.  49,  Bureau  of  Soils, 
U.  S.  Dept.  Agriculture,  1907. 


114  THE   SOU,   SOLUTION 

but  the  reaction  appeared  promptly  when  distilled  water  was 
added,  and  further  examination  showed  the  water  to  contain 
about  0.012  per  cent,  sodium  carbonate.1  Slosson  has  reported 
similar  cases  in  Wyoming.2 

One  "black-alkali"  system  has  been  studied  with  some  approach 
towards  completeness.3  In  this  case  magnesium  and  potassium 
salts  are  not  present,  the  system  being  composed  of  water,  car- 
bon dioxide,  chlorides,  sulphates,  sodium  and  calcium  salts, 
with  the  condition  imposed,  that  the  bases  are  present  in  amounts 
more  than  equivalent  to  the  sulphuric  and  hydrochloric  acids. 
On  desiccation  at  25°  C.  calcium  carbonate  first  appears  fol- 
lowed by  gypsum  and  then  sodium  sulphate  decahydrate.  Next 
appears  a  double  salt  (2CaSO4.3Na2SO4)  followed  by  anhydrous 
sodium  sulphate,  the  Glauber's  salt  which  formerly  crystallized 
being  no  longer  stable.  Sodium  chloride  then  precipitates  and 
the  concentration  finally  reaches  a  point  where  gypsum  is  no 
longer  stable,  and  the  final  group  of  salts  in  contact  with  the 
evaporating  solution  under  conditions  of  stable  equilibrium  con- 
sists of  calcium  carbonate,  the  double  sulphate  of  soda  and  lime, 
anhydrous  sodium  sulphate  and  sodium  chloride. 

The  desiccation  of  a  lake  which  serves  as  the  final  repository 
of  a  regional  drainage  involves  essentially  the  principles  just 
discussed.4  The  constituents  involved  are  the  same.  A  serious 

1  Application  of  the  theory  of  solutions  to  study  of  soils,  by  F.  K. 
Cameron,  Report  No.  64,  Field  Operations  of  the  Bureau  of  Soils,  1899, 
p.  149- 

2  Alkali   lakes   and  deposits,   by   W.    C.   Knight   and   E.   E.    Slosson, 
Bull.  No.  49,  Wyoming  Agr.  Expt.  Station,  1901,  p.  108. 

3  The   solubility   of    certain   salts   present   in   alkali    soils,    by    Frank 
K.   Cameron,  J.   M.  Bell  and  W.   O.   Robinson,  Jour.   Phys.   Chem.,   11, 
396-420  (1907). 

4  It  has  been  suggested  that  the  fact  that  shales  or  similar  geological 
deposits  are  frequently  to  be  found  near  alkali  areas,  indicates  that  the 
shales  are  the  principal  sources  of  the  alkali.     It  is  supposed  that  the 
constituents   of   the   alkali   salts   were    formed   by   the   action   of   water 
on  the  shale  minerals  at  or  about  the  time  the  shales  were  deposited, 
and   carried   down   with   the   latter.      Subsequently   the   alkali   has   been 
leached  out  to  appear  at  the  surface  of  soils,  generally  at  a  lower  level 
than  are  the  shales. 


ALKALI 


problem  involved  in  the  consideration  of  this  source  of  "alkali" 
is  the  high  ratio  of  chlorine  to  the  other  constituents,  in  view 
of  its  very  low  ratio  in  the  rocks  from  which  it  comes.  The 
explanation  undoubtedly  involves  the  fact  that  the  carbonates 
and  sulphates  are  constantly  being  removed  as  calcium  salts  from 
a  body  of  water  which  is  more  or  less  continuously  receiving  the 
drainage  of  any  considerable  watershed,  and  is  at  the  same  time 
subject  to  a  relatively  high  rate  of  evaporation.  The  chlorine 
forming  only  very  soluble  salts  under  such  conditions  would  be 
segregated  and  concentrated  in  the  residual  mother  liquor.  Most 
difficult  is  it  to  account  for  the  relatively  high  ratio  of  sodium  to 
potassium  in  alkali  from  such  an  origin.  Some  light  is  thrown 
on  the  subject  by  the  progressive  changes  in  concentration  of  a 
lake  water  which  receives  a  regional  drainage  under  arid  condi- 
tions. To  this  end  are  given  the  following  results  of  analyses 
of  the  waters  of  Utah  Lake,  made  at  different  times1  over  an 
interval  of  twenty  years,  and  showing  that  there  is  a  segrega- 
tion of  chlorine  and  sodium  taking  place,  although  in  this  case 
the  lake  has  an  outlet  in  the  Jordan  River. 

ANALYSES  OF  THE  WATER  OF  UTAH  LAKE.    RESULTS  IN  PARTS 
PER  MILLION 


Clarke 
1883 

Cameron 
1899 

Brown 
1903 

Seidell 
19042 

Brown 

19048 

Ca  

cc  R 

67  6 

80 

f\7  7 

fa 

<5r  .  . 

°7 

Mf  .  . 

18  6 

11  8 

O2 

77  <; 

86 

•"-••&  
Na  

17  7  ) 

2/17 

J£    

17-7 

233?7  | 

2?  8 

-*o° 

Ti 

•      ) 

n  1 

<>O     , 

•jfis 

778 

ci  

12  A. 

116  ? 

6°J 

oo-t-y 
288  5 

37° 

HCO  .  . 

266 

061 

ro  .  . 

60  Q 

21   7 

mo  . 

58 

Total  

106  o 

892  o 

IAl6 

16o6 

1  The  water  of  Utah  Lake,  by  F.  K.  Cameron :   Jour.  Am.  Chem.  Soc., 
27,  113-116  (1905). 

2  Sample  collected  May  18.     Lake  unusually  high. 

3  Sample  collected  Aug.  31.     Lake  still  high  for  that  season  of  the 
year. 


Il6  THE  SOIL,  SOLUTION 

The  third  general  origin  of  alkali  supposes  that  wind-borne 
sea-spray  carries  into  the  air  salts  which  are  left  in  very  fine 
particles  on  the  evaporation  of  the  water,  or  are  deposited  on 
the  ordinary  atmospheric  dust  and  carried  over  the  land;  and 
that  this  dust  is  precipitated  here  and  there  as  may  be  determined 
by  the  various  meteorological  conditions  which  it  encounters. 
All  the  land  surface  is  supposed  to  be  receiving  more  or  less  of 
it  from  time  to  time,  but  in  arid  regions  the  rainfall  and  drainage 
is  not  sufficient  to  return  to  the  sea  as  much  as  is  received  there- 
from.1 

It  is  very  probable  that  wind-borne  salts  from  the  sea  are 
being  carried  over  and  to  some  extent  being  deposited  on  all  the 
land  surfaces  of  the  earth.  To  what  extent  this  process  is  tak- 
ing place,  and  whether  it  is  sufficient  to  account  for  the  alkali 
of  any  particular  region,  available  data  fail  to  answer  satisfac- 
torily. Probably  it  is  always  associated  with  one  of  the  origins 
of  alkali  already  discussed  and  is  in  itself  generally  of  second- 
ary importance. 

An  argument  frequently  advanced  against  the  validity  of  the 
hypothesis  that  wind-borne  sea-spray  is  the  origin  of  alkali  is 
that  the  relative  proportions  of  the  several  constituents  in  "alkali" 
are  seldom  if  ever  those  obtaining  in  sea  water.  This  argu- 
ment does  not  take  into  consideration,  however,  that  the  several 
salts  in  the  spray  probably  separate  into  crystals  of  widely  dif- 
ferent size  and  specific  gravities,  and  there  may  well  be  taking 
place  a  selective  or  sorting  action  by  the  wind.  More  important, 
undoubtedly,  is  the  selective  action  taking  place  in  the  soil  itself ; 
it  can  only  be  an  accidental  coincidence  that  the  constituents  of 
alkali  in  any  particular  occurrence  should  have  the  same  quanti- 
tative relations  as  in  the  material  from  which  it  originated,  no 
matter  what  may  have  been  the  nature  of  its  origin. 

In  the  field,  alkali  is  found  in  a  bewildering  array  of  forms 
and  types.  Quite  different  combinations  of  constituents  may  be 
found  in  the  same  field  within  a  few  rods  or  even  a  few  feet, 
1  For  a  recent  interesting  and  valuable  discussion  of  this  subject 
with  reference  to  a  particular  area,  see:  The  origin  of  the  salt  deposits 
of  Rajputana,  by  Sir  Thomas  H.  Holland  and  W.  A.  K.  Christie, 
Records  of  the  Geological  Survey  of  India,  38,  154-186  (1909). 


and  each  case  appears  to  have  a  distinct  origin,  to  be  in  fact  a 
law  unto  itself.  Each  alkali  deposit  represents  generally  the 
resultant  from  a  mixture  of  salt  which  has  been  dissolved  and 
reprecipitated  a  number  of  times,  and  which  while  dissolved  has 
been  seeping  through  the  soil  under  gravitational  forces,  or  has 
been  moving  through  the  soil  as  film  water  under  capillary 
stresses.  In  either  event  the  salt  mixture  has  been  subject  to 
the  power  for  selective  absorption  peculiar  to  the  particular  soil 
mass  through  which  it  has  been  moving.  Re-solution  is  seldom 
an  instantaneous  process,  and  different  rates  of  solution  neces- 
sarily involve  some  separation  of  salts.  Finally  the  alkali  de- 
posit is  usually  so  mixed  with  other  soil  material  that  there 
cannot  be  recognized  the  characteristic  solid  phases  (such,  for 
instance,  as  the  double  sulphates  of  calcium  and  another  base) 
which  serve  as  guides  in  laboratory  studies  and  in  certain  salt 
mines.  Even  if  the  characteristic  salts  are  deposited  in  surface 
soils,  it  is  very  doubtful,  owing  to  their  hygroscopicity,  if  any 
but  gypsum,  halite  and  Glauber's  salt  can  persist  for  any  length 
of  time.  The  alternations  of  temperature  from  night  to  day 
characteristic  of  arid  regions,  with  precipitation  of  dews,  might 
easily  be  expected  to  make  noticeable  and  rapid  changes  in  the 
characteristics  of  any  given  alkali  or  salt  mixture. 

It  is  'not  surprising,  therefore,  that  attempts  to  account  for 
the  genesis  and  present  appearance  of  an  alkali  deposit  by  com- 
parison with  artificial  depositions  of  salt  mixtures,  as  worked 
out  in  the  laboratory,  have  generally  been  disappointing.  On 
the  other  hand,  laboratory  studies  have  been  quite  fruitful  in 
elucidating  the  phenomena  taking  place  on  the  leaching  of  alkali 
from  a  soil,  or  so-called  "alkali  reclamation." 

Whatever  the  origin  of  the  alkali,  its  segregation  at  or  near 
the  surface  of  the  soil  is  everywhere  much  the  same;  that  is, 
there  is  a  translocation  and  segregation  of  soluble  salts  in  the 
below-surface  seepage  waters,  determined  mainly  by  the  topo- 
graphic features,  but  partly  by  the  texture  and  structural  prop- 
erties of  the  soil  and  subsoil,  with  a  subsequent  rise  as  capillary 
water  consequent  upon  evaporation  at  the  surface.  Precipita- 
tion of  the  solutes  may  take  place  at  the  surface ;  more  commonly 


Il8  THE  SOIL  SOLUTION 

it  takes  place  a  few  inches  below,  owing  to  the  fact  that  under 
conditions  of  rapid  evaporation,  there  is  ordinarily  a  discon- 
tinuance in  the  capillary  columns  or  the  film  water  at  a  point 
below  the  surface  of  the  soil,  the  water  diffusing  thence  into  the 
above-surface  atmosphere  as  the  vapor  phase. 

The  composition  of  alkali  is  varied.  In  the  vast  majority  of 
cases,  the  world  over,  the  predominating  compound  is  sodium 
chloride.  When  calcium  carbonate  is  a  conspicuous  component 
of  the  soil,  as  a  hard-pan  or  otherwise,  sodium  carbonate  or 
black  alkali  is  also  generally  present,  or  apt  to  appear  when  the 
land  is  irrigated.  When  calcium  sulphate  or  gypsum  is  likewise 
present,  there  is  less  probability  of  appreciable  amounts  of  black 
alkali,  and  where  gypsum  predominates  or  the  calcium  carbo- 
nate is  present  in  relatively  inappreciable  amounts,  black  alkali 
is  generally  absent,  and  sodium  sulphate  is  an  important  con- 
stituent of  the  alkali.  Relative  rates  of  diffusion,  selective  ab- 
sorption, and  sometimes  other  factors  are  prominent,  however, 
and  the  character  of  the  alkali  in  different  spots  within  a  few 
yards  of  one  another  may  differ  greatly.  One  of  the  most  in- 
teresting manifestations  of  alkali  is  the  occasional  occurrence  of 
a  predominating  amount  of  calcium  chloride  which,  as  a  result 
of  its  unusually  high  hygroscopicity,  renders  the  soil  damper, 
and  therefore  darker  in  color  than  the  surrounding  soil,  and 
frequently  causes  even  experts  to  suspect  the  presence  of  black 
alkali.  Its  true  nature  can,  of  course,  be  determined  by  a  sim- 
ple chemical  examination. 

The  effect  of  alkali  on  the  physical  properties  of  the  soil  is 
often  very  marked,  aside  from  the  cementing  action  or  hard-pan 
formation  by  the  carbonate  or  sulphate  of  lime.  Black  alkali, 
by  dissolving  and  segregating  the  organic  matter  at  the  surface, 
removes  from  the  lower  soil  layers  the  "humus"  compounds 
which  are  of  enormous  importance  to  the  maintenance  of  a  soil 
structure  favorable  to  plant  growth.  Moreover,  black  alkali 
is  one  of  the  best  of  deflocculating  agents,  and  consequently 
soils  where  it  is  a  noticeable  component,  frequently  puddle  with 
great  readiness  and  are  reclaimed  with  the  utmost  difficulty. 
Most  of  the  other  constituents  of  alkali,  however,  are  flocculat- 


ALKALI  119 

ing  or  "crumbing"  agencies,  and  if  not  present  in  too  large 
amounts  tend  to  increase  the  readiness  with  which  the  soil  can 
be  brought  into  good  tilth.  In  this  latter  case,  by  separating  in 
the  solid  phase,  or  in  forming  a  viscous  soil  solution,  near  the 
saturation  point,  they  sometimes  produce  a  condition  in  the  soil 
simulating  puddling,  and  where  it  occurs  below  the  surface,  called 
an  alkali  hard-pan. 

The  management  of  soils  infested  with  alkali  is  possible  in 
accordance  with  a  few  well  established  principles.  Substantial 
progress  has  been  made  in  selecting  and  breeding  plants  and 
strains  of  plants  adapted  to  such  soils.  Extreme  cases  are  the 
use  of  the  so-called  Australian  salt-bushes  as  forage  crops,  and 
the  growing  of  date-palms  which  through  generations  of  breed- 
ing in  the  oases  of  the  Sahara  can  thrive  in  lands  so  salty  as 
to  destroy  most  of  the  halophilous  plants.  More  interesting  is 
the  unwitting  development  of  the  farmers  of  Utah  of  strains  of 
wheat  and  alfalfa  which  easily  withstand  three  or  four  times  as 
high  a  salt  content  in  the  soil  as  do  corresponding  crops  in  other 
alkali  regions,  such  as  New  Mexico  and  Arizona.1  Black  alkali, 
or  one  in  which  sodium  carbonate  is  a  prominent  constituent, 
is  especially  destructive  to  vegetation,  not  alone  on  account  of  a 
toxic  action  on  plants,  but  because  in  any  considerable  concen- 
tration it  has  a  corrosive  action  on  the  plant  tissue.  Not  only 
on  this  account  but  also  because  of  its  unfortunate  effects  on 
the  physical  properties  of  the  soil,  black  alkali  has  received  un- 
usual attention  from  soil  investigators.  Hilgard2  has  repeated- 
ly urged  the  use  of  gypsum  as  an  "antidote"  to  black  alkali,  as- 
suming that  under  conditions  of  good  drainage  and  aeration  a 

1  Some    mutual    relations    between    alkali    soils    and    vegetation,    by 
Thomas  H.  Kearney  and  Frank  K.  Cameron,  Report  No.  71,  U.  S.  Dept. 
Agriculture,  1902;    The  date-palm  and  its  utilization  in  the  Southwestern 
states,  by  Walter  T.  Swingle,  Bull.  53,  Bureau  of  Plant  Industry,  U.  S. 
Dept.  Agriculture,    1904;    The   comparative   tolerance   of   various   plants 
for  the  salts  common  in  alkali  soils,  by  T.  H.  Kearney  and  L.  L.  Harter, 
Bull.    113,    Bureau    of    Plant   Industry,    U.    S.    Dept.    Agriculture,    1907; 
Tolerance  of  alkali  by  various  cultures,  by  R.  H.  Loughridge,  Bull.  133, 
California  Agr.  Expt.  Sta.,  1901. 

2  Soils,  by  E.  W.  Hilgard,  1906,  pp.  457-458. 


I2O  THE   SOIL   SOLUTION 

reaction  takes  place  in  accordance  with  the  following  equation, 

Na2CO3  +  CaSO4  =  CaCO3  +  Na2SO4. 

Furthermore,  it  has  been  shown  that  calcium  salts  and  especially 
calcium  sulphate  exercise  a  marked  ameliorating  effect  on  the 
action  of  other  salts  upon  growing  vegetation.1  On  the  other 
hand,  the  reaction  indicated  by  the  equation  just  given  does  not 
run  to  an  end  with  complete  precipitation  of  the  carbonate,  and 
the  total  amount  of  alkali  is  increased  in  the  soil  by  the  addi- 
tion of  the  gypsum.  Unfortunately,  Hilgard's  suggestion  has 
not  yet  acquired  the  sanction  of  satisfactory  field  demonstration, 
although  it  would  seem  to  merit  more  consideration  than  has 
been  given  it.  Inasmuch  as  lime  is  generally  a  prominent  con- 
stituent of  soils  containing  black  alkali,  it  is  possible  that  the 
maintenance  of  good  drainage  and  aeration  in  the  soil  is  itself 
the  best  corrective  of  black  alkali. 

The  best  use  of  alkali  soils  involves  irrigation,  and  it  is  in 
the  application  of  irrigation  waters  that  management  of  alkali 
soils  finds  its  most  highly  developed  and  most  important  ex- 
pression. With  light  sandy  soils  it  has  sometimes  been  found 
practicable  to  add  sufficient  water  to  carry  the  alkali  down  into 
the  soil  to  such  a  depth  that  the  crop  is  well  advanced  toward 
maturity  before  the  alkali  again  rises  in  sufficient  amounts  to 
prove  seriously  detrimental  to  the  more  advanced  crops  which  are 
generally  far  more  "alkali  resistant"  than  the  young  seedlings 
or  the  germinating  seeds.  In  some  cases  this  procedure  can  be 
practiced  for  a  number  of  years  without  greatly  increasing  the 
seriousness  of  the  alkali  conditions,  and  it  may  be  justified,  for 
a  time  at  least,  by  economic  considerations.  Ultimately,  how- 
ever, and  more  quickly  with  heavy  than  with  light  soils,  increas- 
ing amounts  of  alkali  must  be  brought  into  the  surface  soil,  and 
this  method  of  irrigating  should  not  be  considered  as  anything 
more  than  a  temporary  expedient.  The  only  procedure  which 
1  With  the  salts  occurring  in  alkali,  it  is  a  generality  that  the  effects 
produced  on  higher  green  plants  are  relatively  less  wjtih  mixtures  than 
with  an  equivalent  amount  of  a  single  salt.  It  has  recently  been  shown, 
however,  that  the  contrary  is  true  for  at  least  some  kinds  of  bacterial 
flora.  See,  On  the  lack  of  antagonism  between  certain  salts,  by  C.  B. 
Lipman,  Bot.  Gaz.,  49,  41-50  (1910). 


121 

should  be  seriously  considered  as  a  permanent  system  on  an  alkali 
soil,  no  matter  what  the  texture,  is  the  installation  of  under- 
ground drains,  for  which  purpose,  so  far,  cylindrical  tile  drains 
commend  themselves  as  giving  the  best  results.  With  a  well 
established  system  of  tile  drains,  the  alkali  and  all  excess  of 
soluble  salts  can  be  removed  from  the  soil  above  the  drains; 
and  alkali  rising  from  the  soil  below  can,  at  least  very  largely, 
be  prevented  from  rising  to  the  upper  soil  layers.  The  reclama- 
tion of  an  alkali  tract  by  underdrainage  is  not,  however,  a  nec- 
essarily quick  operation.  Generally  it  must  be  a  matter  of  sev- 
eral years  persistent  and  careful  effort,  but  once  attained  should 
readily  be  maintained.  The  reclamation  of  an  alkali  tract  by 
flooding  and  underdrainage  involves  the  reverse  process  to  the 
crystallization  of  salt  from  a  brine.  If  the  water  in  percolating 
through  the  soil  were  long  enough  in  contact  with  the  salts 
present  to  become  a  saturated  solution  in  equilibrium  with  them, 
then  the  composition  of  the  resulting  solution  or  drainage  water 
would  depend  upon  the  particular  solid  phases  or  salts  which 
are  present  in  the  soil,  but  not  on  the  amounts  of  these  salts; 
and  the  relative  proportions  of  the  mineral  constituents  in  the 
drainage  water  should  remain  constant  until  some  one  of  the 
solid  phases  in  the  soil  permanently  disappears. 

In  practice,  however,  the  water  passes  through  the  soil  at 
different  rates  from  time  to  time,  the  flow  from  the  tiles  being 
copious  after  a  flooding  but  gradually  diminishing  as  time  goes 
on.  One  or  both  of  two  processes  can  therefore  take  place. 
The  water  may  dissolve  some  of  the  salts  without  at  any  time 
or  place  becoming  saturated.  As  the  different  salts  have  differ- 
ent rates  of  solution  as  well  as  different  absolute  solubilities, 
it  would  be  expected  that  not  only  the  concentration  of  the 
drainage  water,  but  the  composition  of  the  dissolved  salts  would 
change  from  time  to  time.  On  the  other  hand,  a  part  of  the 
water  may  be  imagined  to  percolate  slowly  through  the  finer 
openings,  thus  forming  a  saturated  solution  with  respect  to  the 
alkali  salts  which  solution,  however,  will  be  diluted  on  entrance 
to  the  drains  by  a  part  of  the  water  going  through  the  larger 
soil  openings  and  dissolving  but  little  salt  in  its  passage.  In 
9 


122  THE  SOIL  SOLUTION 

this  case,  it  would  be  anticipated  that  the  concentration  of  the 
drainage  water  would  increase  as  the  amount  of  flow  diminished 
but  the  composition  of  the  dissolved  salts  would  remain  prac- 
tically constant  until  some  one  or  more  of  the  alkali  salts  was 
completely  removed.  There  are,  unfortunately,  but  few  experi- 
mental data  by  which  these  can  be  tested.  In  the  accompanying 
table  are  given  the  results  of  an  investigation  on  the  reclamation 
of  an  alkali  tract  near  Salt  Lake  City,  Utah,  where  observations 
on  the  composition  of  the  drainage  water  were  made  at  fre- 
quent intervals  for  more  than  three  years.1 

At  first  sight  these  results  might  appear  to  show  that  the  com- 
position of  the  salts  was  remaining  reasonably  constant.  This 
conclusion  must  be  received  with  caution,  however.  Variations 
do  occur  in  the  constituents  which  are  present  in  smaller  amount, 
but  the  variations  are  not  systematic  and  may  plausibly  be  ex- 
plained by  dilution  of  saturated  solution  by  unsaturated  solu- 
tion on  entering  the  drains.  Confining  attention  therefore  to 
the  constituents  occurring  in  larger  proportions,  namely,  sodium 
chloride,  sodium  sulphate  and  sodium  bicarbonate  (including 
the  normal  carbonate)  it  should  be  remembered  that  the  per- 
centage of  sodium  in  these  three  salts  does  not  vary  much,  and 
the  "constancy"  may  be  more  apparent  than  real.  Indeed  a 
close  inspection  of  the  results  indicates  that  while  the  sodium 
is  remaining  practically  unchanged,  there  is  some  decrease  in 
the  chlorine  and  a  corresponding  increase  in  the  sulph-ion.  From 
this  it  would  follow  that  the  sodium  chloride  was  being  washed 
out  of  the  soil  more  rapidly,  proportionately,  than  sodium  sul- 
phate; and  it  would  also  appear  that  the  solution  entering  the 
drains  was  not  in  final  equilibrium  with  the  salts  in  the  soil. 

How  long  drainage  must  continue  before  there  is  a  radical 
change  in  the  composition  of  the  seepage  water  cannot  be  pre- 
dicted, and  unfortunately  data  regarding  this  point  are  not  avail- 
able. It  is  certain  that  in  time  some  one  or  more  of  the  salts 
in  the  soil  would  be  removed  and  the  nature  of  the  drainage 
1  See,  Calcium  sulphate  in  aqueous  solution,  by  Frank  K.  Cameron 
and  James  M.  Bell,  Bull.  No.  33,  1906,  pp.  10  and  70,  and  Reclamation 
of  alkali  land  in  Salt  Lake  Valley,  Utah,  by  Clarence  W.  Dorsey,  Bull 
No.  43,  1907,  p.  13,  Bureau  of  Soils,  U.  S.  Dept.  Agriculture. 


ALKALI 


123 


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124  THE   SOII<   SOLUTION 

water  would  be  changed.  Alterations  in  the  composition  of  the 
drainage  water  furnish  the  readiest  as  well  as  the  best  guides 
as  to  the  changes  and  the  nature  of  the  changes  taking  place 
in  the  soil  during  the  process  of  reclamation.  As  a  practical 
matter  it  should  be  borne  in  mind  that  the  persistence  of  the 
several  salts  of  the  alkali  mixture  does  not  mean  necessarily 
that  they  are  evenly  distributed  in  the  soil ;  while  yet  determining 
the  composition  of  the  water  entering  the  drain,  they  may  have 
disappeared  from  the  upper  soil  layers  which  then  may  hold  a 
solution  of  quite  different  character,  suited  to  the  support  of 
crops.  In  the  case  just  cited  the  soil  contained,  before  drainage 
operations  were  commenced,  upwards  of  2.7  per  cent,  of  readily 
soluble  salts  and  would  not  support  any  growth  other  than  salt- 
bushes  and  similar  halophilous  plants.  Four  years  later  the  soil 
contained  less  than  0.3  per  cent,  soluble  salts  and  yielded  a  very 
satisfactory  crop  of  alfalfa.  In  such  cases,  however,  the  land 
cannot  be  considered  as  finally  reclaimed  until  a  material  change 
in  the  composition  of  the  drainage  water  shows  that  there  has 
been  a  complete  removal  of  some  of  the  solid  salts  from  that 
portion  of  the  soil  feeding  the  drains. 

The  rate  at  which  alkali  can  be  leached  from  a  soil  is  de- 
pendent in  a  large  measure  upon  the  absorptive  properties  of 
the  soil,  and  to  some  extent  upon  the  nature  of  the  salts  com- 
posing the  alkali.  The  leaching  is  more  rapid  from  sandy  than 
from  clay  soils,  and  white  alkali  is  leached  more  readily  than 
is  black.  In  general,  however,  the  same  laws  hold  here  as  in 
any  leaching  of  a  solute  from  an  absorbent,  and  it  has  been  shown 
that  even  in  the  case  of  black  alkali,  the  rate  of  removal  under 
a  constant  leaching  follows  the  law  dx/dt  =  K  (A  —  x).1  In 
practice,  the  water  does  not  percolate  through  the  soil  under  a 
constant  "head,"  but  the  flow  is  intermittent,  so  that  the  value 
of  the  above  formula  is  mainly  academic.  On  the  other  hand, 
if  the  drainage  between  floodings  is  thorough,  this  procedure 
should  be  more  efficient  than  any  other  for  causing  a  rapid  re- 
moval of  the  alkali  salts,  if,  as  is  generally  the  case,  a  limited 
quantity  of  water  is  available. 

1  The  removal  of  "black  alkali"  by  leaching,  by  F.  K.  Cameron  and 
H.  E.  Patten,  Jour.  Am.  Chem.  Soc.,  28,  1639  (1906). 


ALKALI  125 

Finally,  it  remains  to  be  pointed  out  that  the  use  of  exces- 
sive amounts  of  water  on  alkali  tracts  is  quite  as  unfortunate  in 
its  effects  as  the  use  of  too  little.  If  water  be  added  to  an  un- 
drained  soil  or  in  excess  of  the  capacity  of  the  drains  to  remove 
it,  incalculable  harm  may  be  done  by  enormously  increasing 
in  the  surface  soil  the  amount  of  salts  brought  up  from  the  lower 
layers  as  the  capillary  stream  rises  to  the  surface  in  consequence 
of  evaporation  there.  Should  the  wetting  of  the  soil  proceed  so 
far  as  to  establish  good  capillary  connection  with  the  permanent 
ground  water,  the  harm  may  be  sufficient  to  offset  in  a  few  weeks 
or  months  expensive  reclamation  efforts  of  years.  The  harm  to 
the  tract  where  the  water  is  added  may  be  far  less  than  the  harm 
done  to  other  areas.  A  large  proportion  of  existing  alkali  deposits 
or  "spots"  results  from  the  evaporation  of  seepage  waters  coming 
sometimes  from  considerable  distances.  The  overwetting  of  a  soil 
means  the  production  of  seepage  waters  which  are  to  appear  at 
the  surface  somewhere  else,  generally  at  a  lower  level,  and  fre- 
quently means  the  more  or  less  complete  ruin  of  the  soils  of  the 
lower  level.  The  experience  of  India,  Africa  and  our  own  arid 
states  in  the  increase  of  alkali  spots  following  the  introduction  of 
irrigation,  added  to  our  present  theoretical  knowledge,  should 
make  the  planning  of  an  irrigation  project  without  adequate  drain- 
age provisions,  a  stupidity,  and  its  accomplishment  a  public  crime. 
Quite  as  important  is  the  development  of  a  public  opinion  that  the 
individual  cultivator  who  deliberately  or  carelessly  uses  excessive 
amounts  of  water  on  his  tract  is  a  serious  enemy  to  the  body 
politic,  and  should  be  treated  as  such. 


INDEX. 

PAGE 

Absorbents,  Influence  on  soil  extracts 38 

Absorption  by  soils  9,  59,    65 

formula   62 

of  dyes  60,    61 

rate    63 

selective    61 

Acid  digestion  of  soils u,     12 

Adsorption    9,     60 

Alkali    1 10,  1 18 

Effect  on  soils 118 

Order  of  deposition 112 

Reclamation    117,  121 

Source    in,  117 

Antagonism  between  salts 120 

Apophyllite,  Crystallization  from  water 35 

Apple  trees,  Effect  of  grass  on 98 

Appleyard,  James  R.    See  Walker,  James,  and  Appleyard,  James  R. 

Ash  analyses 1 1,     13 

Association  of  Official  Agricultural  Chemists'  analyses,  quoted 12 

cited    12 

"official   method". ..  .10,     12 

"Available"  and  "non-available"  plant-food  elements 8 

Averitt,  S.  D.    See  Peter,  Alfred  M.,  and  Averitt,  S.  D. 

Bacteria  in  soils   103 

Bailey,  Liberty  H.,  cited , 5 

Balance  between  supply  and  removal  of  mineral  plant  nutrients 75 

Barium  in  soils    107 

Bardt,  A.    See  Doroshevskii,  A.,  and  Bardt,  A. 

Becquerel,  Antoine  C.,  cited 67 

quoted    68 

Bell,  James  M.,  and  Cameron,  Frank  K.,  cited 28 

Bell,  James  M.     See  also  Cameron,  Frank  K.,  and  Bell,  J.  M. ;  Cam- 
eron, Frank  K.,  Bell,  J.  M.,  and  Robinson,  W.  O. 

Benedick,  Carl,  cited    55 

Birner,  H.,  and  Lucanus,  B.,  cited 70 

Bischof,  Gustav,  cited   113 

Black  alkali no,  114,  119,  124 

Blanck,  Edward,  cited   63 

Breazeale,  James  F.,  acknowledgments 80 

cited    71 

See   also    Cameron,    Frank    K.,    and    Breazeale, 
J.  F. ;    LeClerc,  J.  A.,  and  Breazeale,  J.  F. 


128  INDEX 


'  PAGE 
Briggs,  James  J.,  cited 55 

and  Lapham,  Macy  H.,  cited 41 

and  McLane,  John  W.,  cited 26 

Martin,  F.  O.,  and  Pearce,  J.  R.,  cited 31 

Brooks,  William  P.,  cited 5 

Brown,  Bailey  E.,  cited 46 

quoted    46,  115 

Bryan,  H.    See  Davis,  R.  O.  E.,  and  Bryan,  H. 

Buckingham,  Edgar,  cited   ; 30 

Burney,  W.  B.,  quoted 98 

Cameron,  Frank  K.,  cited no,  114,  115 

See  also  Bell,  James  M.,  and  Cameron,  Frank  K. ; 
Kearney,  Thomas  H.,  and  Cameron,  Frank  K. ; 
Whitney,  Milton,  and  Cameron,  Frank  K. 

and  Bell,  James  M.,  cited 31,  38,  50,  113,  122 

and  Breazeale,  James  F.,  cited 62 

and  Gallagher,  Francis  E.,  cited 24 

and  Patten,  Harrison  E.,  cited 63,  124 

and  Robinson,  William  O.,  cited 27,     53 

Bell,  James  M.,  and  Robinson,  William  O.,  cited  114 

Calcium  nitrate,  basic   108 

Carbon  dioxide  in  the  soil 53 

Charpentier,  Jean  G.  F.,  cited 113 

Chemical  analysis  of  soils.     See  Soil  analysis — Chemical. 

Chesneau,  G.,  cited   68 

Christie,  W.  A.  K.    See  Holland,  Sir  Thomas  H.,  and  Christie,  W.  A.  K. 

Clarke,  Frank  Wigglesworth,  cited 76,  115 

Coff ey,  George  N.,  quoted 23 

Concentration  of  mineral  constituents 39 

Concentration,  Plant  growth  and 70 

Cracking  of  soil   22 

Creep    19 

Creighton,   Henry   J.    M.     See   Findlay,   Alexander,    and    Creighton, 
Henry  J.  M. 

Critical  moisture  content  24 

Crop  control  methods  7,  105 

plants  defined  I 

producing  power  and  aqueous  extract 81 

rotation,  Natural   97 

Obj  ects  of  4 

yields  increasing  16 

Crumb  structure  of  soils 25 

Crumbing 27,  119 


INDEX  129 

PAGE 

Cushman,  Allerton  S.,  cited 36 

"Cut-off"    22,  75 

Cyanamid 108 

Czapek,  Friedrich,  Experiments  on  root  etchings 9 

Criticism  of  Molisch 101 

Dachnowski,  Alfred,  cited 88 

Darbishire,  Francis  V.,  and  Russell,  Edward  J.,  cited 103 

Darwin,  Horace,  cited 22 

Davis,  R.  O.  E.,  quoted 63 

and  Bryan,  H.,  cited 55 

De  Candolle,  Augustin  P.,  cited 97 

Degradation  of  rocks  I 

De  Roode,  Rudolph  J.  J.,  quoted 98 

Diaspore   34 

Dittrich,  Max.,  cited 13 

Doroshevskii,  A.,  and  Bardt,  A.,  cited 35 

Dorsey,  Clarence  W.,  cited no,  122 

Drainage  waters,  Composition   124 

Drought  limits  defined   29 

Dunnington,  Francis  P.,  cited 98 

Dust   20 

Dyer,  Bernard,  cited 40 

method  of  soil  analysis 10 

quoted    6 

Dynamic  nature  of  soil  phenomena 18 

Earthworms    22 

European  soils,  analyses   16 

Erosion   20 

Etchings,  Root   9 

Ewart,  A.  J.,  cited 18,  72,  73 

Excreta,  Toxic  99,  100,  103 

"Factors"    1 1 

Failyer,  George  H.,  cited 107 

See  also  Schreiner,  Oswald,  and  Failyer,  George  H. 

Smith,  Joseph  G.,  and  Wade,  H.  R.,  cited 32 

"Fairy  rings"   98 

Feldspars  35,  38,  55 

Fertilizers   4,  83,  105 

Film  water  24 

tenacity,  Experiments   25 

Findlay,  Alexander,  and  Creighton,  Henry  J.  M.,  cited 53 

Fine  a  soil,  to 4 

Fischer,  Emil,  and  Schmidmer,  Edward,  cited 61 

"Fly-off"    22,     75 


130  INDEX 

PAGE 

Frear,  William,  cited 5 

Free,  Edward  Elway,  cited 20 

Friedel,  Charles,  and  Sarasin,  Edmond,  cited 34 

Gallagher,  Francis  Edward.    See  Cameron,  Frank  K.,  and  Gallagher, 
Francis  E. 

Gannett,  Henry,  cited  76 

Gaudechon,  H.    See  Muntz,  A.,  and  Gaudechon,  H. 

Geikie,  Sir  Archibald,  cited 75 

Gels    36 

Gilbert,  Joseph  H.,  cited 98 

Gonnard,  F.,  cited  35 

"Good"  and  "poor"  soils  compared 80 

Graham,  Thomas,  cited  67 

Granulate  a  soil,  to 4 

Grass,  Effect  on  apple  trees 98 

Gravitational  water  23 

Great  Salt  Lake,  Reaction  of  water 113 

Green  manure,  Effect  on  soil  extracts 87 

Gypsum  on  alkali  soils 119 

Hardpan   HI 

Harter,  Leonard  L.    See  Kearney,  Thomas  H.,  and  Harter,  L.  L. 

Hartwell,  Burt  L.,  Wheeler,  H.  J.,  and  Pember,  F.  R.,  cited 74 

Haselhoff,  Emil.    See  Konig,  Joseph,  and  Haselhoff,  E. 

Haworth,  Erasmus,  cited  113 

Heileman,  William  H.,  quoted 65 

Heterogeneity  of  soils i,  21,  32,  79 

Hilgard,  Eugene  W.,  cited 5,  6,  38,  40,  1 19 

Method  of  soil  analysis 10 

Hillebrand,  William  F.,  cited 13 

Hills,  Joseph  L.,  cited 5 

Holland,  Sir  Thomas  H.,  and  Christie,  W.  A.  K.,  cited 116 

Hulett,  George  A.,  cited 68 

Humic  acids  55 

Humus    6 1 

Hutchinson,  Henry  B.    See  Russell,  Edward  J.,  and  Hutchinson,  Henry  B. 

Hydrolysis    33 

Imbibition    55 

Irrigation    120 

Johnson,  Samuel  W.,  cited 40,  77 

quoted    2 

Kahlenberg,  Louis,  and  Lincoln,  Azariah  T.,  cited 35 

Kaolinite    34 

Kearney,  Thomas  H.,  and  Cameron,  Frank  K.,  cited 119 

and  Harter,  Leonard  L.,  cited , 119 


INDEX  131 

PAGE 

Kentucky  agricultural  experiment  station,  Method  of  soil  analysis ...  10 

King,  Franklin  H.,  cited  75,  76,  77 

quoted  46,  76 

Knight,  Wilbur  C.,  and  Slosson,  Edwin  E.,  cited 114 

Konig,  Joseph,  and  Haselhoff,  E.,  cited 8 

Kossovich,  Petr.  S.,  Experiments  on  root  etchings 9 

Lagergren,  Sten,  cited  26 

Lake  desiccation  114 

Lapham,  Macy  H.    See  Briggs,  Lyman  J.,  and  Lapham,  Macy  H. 
Lawes,  John  B.,  and  Gilbert,  Joseph  H.    See  Gilbert,  Joseph  H. 

Leather,  J.  Walter,  cited 23 

Le  Clerc,  J.  Arthur,  and  Breazeale,  James  F.,  cited 14 

Lemberg,  Johann  T.,  cited 35 

Liebig,  Justus,  cited   8,  97 

Liebrich,  A.,  cited  34 

Liebreich,  quoted  68 

Lieving,  quoted   68 

Lipman,  Jacob  G.,  cited 72,  103 

See  also  Voorhees,  Edward  B.,  and  Lipman,  Jacob  G. 

Lipman,  C.  B.,  cited 120 

Livingston,  Burton  E.,  cited 85,  88  97 

Lincoln,  Azariah  T.    See  Kahlenberg,  Louis,  and  Lincoln,  Azariah  T. 

Litmus,  Absorption  of  66 

as  indicator   66 

Loughridge,  Robert  H.,  cited 28,  119 

Lucanus,  B.    See  Birner,  H.,  and  Lucanus,  B. 

McGee,   W.  J.,  quoted 22,  76 

McLane,  John  W.    See  Briggs,  Lyman  J.,  and  McLane,  John  W. 

Manure,  Stable,  Effect  on  soil  extracts 84 

Martin,  F.  Oskar.    See  Briggs,  Lyman  J.,  Martin,  F.  O.,  and  Pearce,  J.  R. 

Maxwell,  Walter,  Method  of  soil  analysis 10 

Mechanical  analysis    31 

Merrill,  George  P.,  cited 9 

Meyerhoff er,  Wilhelm,  cited   in 

Meyer,  Victor,  cited  67 

Minchin,  George  M.,  cited 26 

Mineral  constituents  of  soil  solution 31,  37 

Mineral  plant  nutrients,  Balance  between  supply  and  removal 75 

Mississippi  River,  Soil-carrying  power 21 

Mixing  of  soils    33 

Moisture  content   24 

Moisture  movement  into  soil 28 

Molisch,  Hans,  cited   101 

Mooers,  Charles  A.,  cited 10 


132  INDEX 

PAGE 

Motion  in  soils  19 

Movement  of  soils   20 

Muntz,  A.,  and  Gaudechon,  H.,  cited 30 

quoted    24 

Murray,  Sir  John,  cited 75 

Newell,  Frederick  H.,  cited 75 

Night  soil  108 

Nitrates  in  agriculture   108 

in  soil  solution  103 

Nitrogen  carriers   103 

"Official  method"  of  soil  analysis 10 

Optimum  moisture  content  24 

Organic  compounds,  Effect  on  plants 82 

Organic  constituents  of  soil  solution 54,  79 

Orthoclase,  Alteration  of  33 

Ostwald,  Wo.,  cited   28 

Oxidizing  power  of  roots 101 

Oxygen  in  the  soil 53 

Oxystearic  acid,  Toxic  to  plants : . . . .  96 

Patten,  Harrison  E.,  cited 24,  25,  60 

See  Cameron,  Frank  K.,  and  Patten,  Harrison  E. 

and  Waggaman,  William  H.,  cited 9,  59 

and  Gallagher,  F.  E.,  cited 59 

Pearce,  Julia  R.    See  Briggs,  Lyman  J.,  Martin,  F.  O.,  and  Pearce,  J.  R. 
Pember,  F.  R.    See  Hartwell,  Burt  L.,  Wheeler,  H.  J.,  and  Pember,  F.  R. 

Penfield,  Samuel  L,.,  cited 13 

Percolation  experiments    47 

Peter,  Alfred,  cited  54 

and  Averitt,  S.  D.,  cited 10 

Pfeffer,  Wilhelm  F.  P.,  cited 18,  72,  73,  101 

Phlogiston  theory  17 

Phosphates   50 

Picoline  carboxylic  acid,  toxic  to  plants 96 

Plant- food  theory  16 

Plant  growth  and  concentration 70 

Plant  nutrients,  Supply  and  removal 75 

Plot  experiments   14 

"Poor"  and  "good"  soils  compared 80 

Pot  experiments  14 

Puddling    25 

Pyrogallol    87 

Pyrophyllite   34 

Ragweed    97,  98 

Rainfall    22,  75 


INDEX  133 

PAGE 

Rajputana,  Salt  deposits 116 

Rayleigh,  Lord,  cited  26 

Reed,   Howard   S.     See  Schreiner,  Oswald,  and   Reed,  Howard   S. ; 
Schreiner,  Oswald,  Reed,  Howard  S.,  and  Skinner,  J.  J. 

Removal  of  plant  nutrients,  Supply  and 75 

Reversible  reactions   34 

Ries,  Heinrich,  quoted  112 

River  waters,  Concentration  of 76 

Robinson,  William  O.    See  Cameron,  Frank  K.,  and  Robinson,  William 
O. ;  Cameron,  Frank  K.,  Bell,  James  M.,  and  Robinson,  W.  O. 

Rodewald,  H.,  cited   24 

Romer,  Hermann.     See  Wilfarth,  Hermann,   Romer,  Hermann,  and 
Wimmer,  G. 

Root  etchings   9 

Root  growth  mechanism   19 

Roots  of  growing  plants 18 

Rotation  of  crops   97 

Rothmund,  V.,  cited   68 

"Run-off"    22,  75 

Russell,  Edward  J.,  cited 103 

See   also    Darbishire,    Francis   V.,    and    Russell, 
Edward  J. 

and  Hutchinson,  Henry  B.,  cited 72 

Sachs,  Julius,  Experiments  on  root  etchings 9 

Salt  as  fertilizer,  Common 108 

Sarasin,  Edmond.    See  Friedel,  Charles,  and  Sarasin,  Edmond 34 

Schmidmer,  Edward.    See  Fischer,  Emil,  and  Schmidmer,  Edward. 

Schreiner,  Oswald,  quoted   102 

and  Failyer,  George  H.,  cited 41,  47 

and  Reed,  Howard  S.,  cited 100,  101 

and  Shorey,  Edmund  C.,  cited 95 

and  Sullivan,  M.  X.,  cited 100 

Reed,  Howard  S.,  and  Skinner,  J.  J.,  quoted 89 

Sea  water,  Desiccation  of ill 

Seedlings,  Growth  of 74,  80,  82,  84,  86,  88,  100,  102 

Seedlings,  Toxic  action  of  acids  and  salts 62 

Seidell,  Atherton,  quoted   115 

Shaler,  Nathaniel  S.,  cited 20 

Shorey,  Edmund  C.,  cited 95 

See  also  Schreiner,  Oswald,  and  Shorey,  E.  C. 

Shrinking  of  soils 22 

Skinner,  J.  J.,  quoted 99,  102 


134  INDEX 

PAGE 
Skinner,  J.  J.     See  also  Schreiner,  Oswald,  Reed,  Howard  S.,  and 

Skinner,  J.  J. 
Slosson,  Edwin  E.    See  Knight,  Wilbur  C.,  and  Slosson,  Edwin  E. 

Smith,  Joseph  G.,  quoted 98 

See  also  Failyer,  George  H.,  Smith,  Joseph  G.,  and 
Wade,  H.  R. 

Sodium  chloride  as  fertilizer 108 

Soil,  the  I 

Soil  amendments  105 

analysis,  Chemical  8,  22 

Methods    10 

atmosphere    23 

bacteria   23,  103 

control  4 

methods    4 

erosion    20 

fatigue    100 

heaving   22 

individuality  2 

management   '. 2,  3,  4 

minerals,  Chief  32 

moisture  defined  i 

not  a  static  system 18 

phenomena,  Dynamic  nature  of 18 

shrinking    22 

solution  defined  I 

Analyses    39 

Importance  of  2 

Organic  constituent  of 79 

Survey  Field  Book,  cited 3 

translocation  by  water   20 

wind   21 

Soils,  Composition  of I 

Mineral  constituents  of  32 

Moisture  content   24 

Water  extracts  of  39 

Solid  solution  defined 59 

Solubility  of  minerals  .52,  55 

Spring,  Walthere,  cited  67 

Structure    27 

Subsoils,  Infertility  of 88 


INDEX  135 

PAGE 

Sullivan,  Michael  X.,  cited 102 

quoted    68 

See  also  Schreiner,  Oswald,  and  Sullivan,  M.  X. 

Supply  and  removal  of  plant  nutrients 75 

Surface  effects   67 

Surface  tension  27 

Swan  tract,  Utah  123 

Swingle,  Walter  T.,  cited 119 

Taylor,  Frederick  W.,  cited 5 

Tennessee  agricultural  experiment  station,  Methods  of  soil  analysis..  10 

Thorne-,  Charles  E.,  cited 5 

Tillage  methods  4 

Objects  of   4 

Tollens,  Bernhard  C.  G.,  cited 14 

Toxic  excreta  of  roots 99,  100,  103 

Udden,  Johan  August,  quoted 21 

U.  S.  Dept.  of  Agriculture,  Bureau  of  Soils.    See  Soil  Survey  Field 

Book. 

U.  S.  Geological  Survey,  cited 13 

Underdrainage    121 

Utah  Lake  water  analyses 115 

Van  Hise,  Charles  R.,  cited 35,  36 

van't  Hoff,  Jakob  H.,  cited 67,  in 

Voorhees,  Edward  B.,  and  Lipman,  Jacob  G.,  cited 72,  103 

Wade,  Harold   R.     See  Failyer,  George  H.,  Smith,  Joseph  G.,  and 

Wade,  H.  R. 
Waggaman,  William  H.     See  Patten,  Harrison  E.,  and  Waggaman, 

William  H. 

Walker,  James,  and  Appleyard,  James  R.,  cited 60 

Washington,  Henry  S.,  cited 13 

Water,  Movement  into  soils 28 

vapor,  Movement  in  soils 29 

Way,  John  T.,  cited 9 

Weeds,  Analyses  of  98 

Weinschenk,  E.,  cited  35 

Wheeler,  Homer  J.,  cited 74 

Wheeler,  Homer  J.    See  also  Hartwell,  Burt  L.,  Wheeler,  H.  J.,  and 

Pember,  F.  R. 

White  alkali   no,  in 

Whitney,  Milton,  cited  16 

and  Cameron,  Frank  K.,  cited 26,  42 

Wilfarth,  Hermann,  Romer,  Hermann,  and  Wimmer,  G.,  cited 14 


136  INDEX 

PAGE 
Willard,  Julius  T.,  cited 5 

Wimmer,  G.    See  Wilfarth,  Hermann,  Romer,  Hermann,  and  Wimmer,  G. 

Wind    20 

Carrying  power  of 21 

Wind-borne  soil  material  21,  33 

Wohler,  Friedrich,  cited   35 

Wolff,  Emil  T.  von,  tables,  cited 77 

Woburn,  Experiments  at 98 

Young,  Thomas,  cited   26 

Zeolites 9,  34,  35 


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